2028年3月期——日本のサステナビリティ情報が「監査される数字」になる年

Most Japanese boards have the right date in the calendar. It is the wrong one.

The date everyone has is the fiscal year ending March 2027 — when the largest Prime-listed companies must file sustainability disclosures under the SSBJ Standards inside their annual securities reports. It is a real deadline and it is close. But it is not the deadline that changes behaviour.

The one that does is a year later. Under the FSA’s Roadmap on Sustainability Disclosure and Assurance, “mandatory assurance will begin one year after the mandatory application of the SSBJ Standards.” For the first cohort, that means the fiscal year ending March 2028. At that point a third party starts checking the numbers.

Disclosure asks a company what it wants to say. Assurance asks it to prove it.

The Two Dates, and Who Is Standing on Them

The FSA has published the phase-in with unusual precision, including how much of the market each tier represents. From the roadmap as of July 2025:

• Market cap ¥3 trillion and above — disclosure from the fiscal year ending March 2027. 68 companies, 54.1% of TSE Prime market capitalisation.
• ¥1 trillion and above — from the fiscal year ending March 2028. 171 companies, 72.5%.
• ¥500 billion and above — from the fiscal year ending March 2029. 284 companies, 80.8%. The FSA notes the timing for this third tier “may be further discussed.”

For everyone else on Prime, the roadmap says mandatory disclosure is “to be considered” — determined later, based on disclosure practice and investor needs.

Read the percentages rather than the counts. Sixty-eight companies sounds like a narrow rule. Fifty-four percent of the Prime market by value is not a narrow rule; it is most of the investable Japanese equity market. By the third tier, four-fifths of Prime market capitalisation sits inside the regime while fewer than 300 companies are named. Japan chose to regulate market weight, not company count — which means the disclosure floor of the Japanese market moves years before most Japanese companies are technically in scope.

And the assurance clock runs one year behind each of those tiers. The 68 largest disclose for March 2027 and are assured for March 2028. The next 103 disclose for March 2028 and are assured for March 2029.

What “Limited Assurance” Actually Means Here

Two details in the roadmap matter more than the headline.

The first is the level. The FSA states the assurance level is limited assurance, and adds a clause that deserves attention: “possibility of transitioning to reasonable assurance will no longer be considered.” Japan is not treating limited assurance as a waypoint on the road to full audit-grade verification. It is the destination, at least for now.

The second is the scope. For the first two years, assurance covers Scope 1 and Scope 2 emissions, Governance, and Risk Management — with expansion after the third year “considered based on international practices.”

That scope is the tell. Regulators picked the three things that can actually be traced back to something. Scope 1 and 2 emissions resolve to meters, fuel invoices, and power purchase agreements. Governance resolves to board minutes, committee charters, and mandates. Risk Management resolves to a documented process that either exists or does not. Scope 3, targets, and transition plans — the parts of a sustainability report that are hardest to trace to a record — are outside the first window.

In other words, the initial assurance regime is a test of whether your sustainability claims are connected to your operational records. Not whether your ambition is impressive.

Three Things That Break First

The European experience under CSRD, where limited assurance has applied from the first report, suggests where the pressure lands.

Provenance, not calculation. Most teams can calculate an emissions figure. Far fewer can hand an assurance provider the meter reading, the invoice, the conversion factor, the version of the factor used, and the person who approved it — for every number in the disclosure. Assurance is not a maths check. It is a chain-of-custody check, and chain of custody is a systems property that cannot be reconstructed at the end of the year.

Governance claims become testable. “The Board oversees climate-related risks” is a sentence that has appeared in Japanese integrated reports for years without consequence. In the assurance regime it is a claim about a documented process, and someone will ask to see the process. Companies where the sustainability team writes the governance narrative without the corporate secretary in the room are going to have an uncomfortable first year.

The two-report problem. Japan’s transition relief allows sustainability disclosures to be reported after the financial statements for two years, moving to simultaneous disclosure afterwards. Meanwhile the glossy integrated report keeps its own timeline and its own numbers. Companies now maintain a securities-report figure that is assured and a marketing figure that is not, describing the same year. Where those two drift apart, the assured one becomes the record — and the other becomes a question.

Why This Is an Infrastructure Question, Not a Reporting Question

The instinct is to solve this with people: hire another sustainability manager, retain another consultant, run a longer data-collection cycle.

That instinct scales badly, because the underlying problem is not effort. It is that most sustainability data lives in a spreadsheet lineage nobody can reconstruct — a figure copied from a site report into a regional roll-up into a group template, with the reasoning held in someone’s memory. That arrangement survives disclosure. It does not survive assurance, because an assurance provider’s first question is not “what is the number” but “where did it come from.”

Companies that clear this cheaply will be the ones that treat sustainability data the way they already treat financial data: a single source of record, entries that carry their own provenance, controls that run continuously rather than annually, and outputs generated from that record instead of assembled alongside it. That is an engineering brief, and it needs to be commissioned in 2026 to be useful in 2028. A data architecture is not something a company procures in the quarter it is needed.

This is also where the technology question gets genuinely interesting rather than merely administrative. Automated data capture, lineage tracking, and machine-assisted mapping between frameworks are not a productivity story here — they are the difference between a disclosure that survives external examination and one that does not.

What Assured Data Does to Capital

The wider consequence is the reason this belongs on an executive agenda rather than a compliance one.

Sustainability data across the region has been abundant and hard to compare: different boundaries, different methodologies, different vintages, and — until now — no external check. It is not surprising that many investors have discounted it, treating ESG disclosure as narrative rather than as data.

Assurance changes the input. From March 2028, a meaningful share of the Japanese market will publish emissions and governance information that someone independent has examined, on a common standard, inside a regulated filing. That is the precondition for sustainability data being used the way financial data is used — in screens, in models, in covenants, in diligence.

For companies with a genuine operational story, this is an opportunity that did not previously exist, because the claim finally carries evidence a market can price. For companies whose sustainability position is mostly narrative, the same shift removes the cover that narrative provided. The gap between those two groups has always existed. Assurance makes it visible, and it makes it visible on a date that is already set.

This is the argument for treating 2026 as the build year rather than the planning year.

Before March 2027

A short list of what tends to distinguish the companies that arrive prepared:

Work backwards from the assured scope. Scope 1 and 2, Governance, and Risk Management are the first things examined. Get those three to audit-grade provenance before broadening effort across topics that are not yet in scope.

Run a dry-run assurance this year. Ask an assurance provider, or your internal audit function, to test the current process against the published scope. The findings arrive while there is still time to change systems rather than to write explanations.

Name an owner with system authority. Assurance readiness is a data-architecture outcome. If the person accountable for it cannot commission changes to how data is captured across sites and subsidiaries, the accountability is nominal.

Reconcile your two reports now. Put the securities-report figures and the integrated-report figures side by side for the current year and explain every difference. Differences that cannot be explained internally will not be explainable externally.

Read the FSA roadmap directly. It is a single chart. The tiers, the assurance timing, the scope, and the transition relief are all stated plainly on it, and most of the confusion in the market comes from summaries rather than the source.

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Japan is doing something that other markets have mostly done in the opposite order: setting the verification date almost at the same time as the disclosure date, and telling companies exactly which claims will be checked first. That is a gift, if it is read as one. The companies that treat March 2028 as the real deadline have two years to build. The companies that treat March 2027 as the deadline have one year to build and one year to explain.

Tech for Impact Summit 2027 convenes in Tokyo on May 18–19, 2027 — an invitation-only gathering of leaders working at the intersection of technology, capital, and impact, and a partner event of SusHi Tech Tokyo. Executives and investors working on this transition can request an invitation.

Related reading: Beyond Compliance: Why Japan’s Best Companies Are Turning ESG Disclosure into a Competitive Weapon · Impact Investing in Japan: The 2026 Landscape · SusHi Tech Tokyo 2027: The Complete Guide