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

From the fiscal year ending March 2027, the largest Prime-listed companies must file sustainability disclosures under the SSBJ Standards inside their annual securities reports. Most boards have that date in the calendar.

The date that will change how companies operate comes 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 is the fiscal year ending March 2028. From then on, a third party checks the numbers.

The Two Dates, and Who Is Standing on Them

The FSA has published the phase-in with the market share of each tier attached. 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 the rest of Prime, the roadmap says mandatory disclosure is “to be considered,” depending on disclosure practice and investor needs.

Sixty-eight companies is a short list, but together they are 54.1% of Prime market value. By the third tier, fewer than 300 named companies cover four-fifths of Prime capitalisation, while the majority of listed companies are still outside the regime.

Assurance follows each tier by one year. 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” Means Here

Two details in the roadmap carry more weight than the headline.

The level is limited assurance, and the FSA adds that the “possibility of transitioning to reasonable assurance will no longer be considered.” Limited assurance is the end state, at least for now, rather than a step toward audit-grade verification.

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

Each of those three can be traced to a record. Scope 1 and 2 emissions come from meters, fuel invoices and power purchase agreements. Governance comes from board minutes, committee charters and mandates. Risk Management is a documented process that either exists or does not. Scope 3, targets and transition plans, the hardest parts of a sustainability report to trace to anything, are left out of the first window.

Three Things That Break First

CSRD in Europe has applied limited assurance from the first report, and the experience there shows where the pressure lands.

Provenance. Most teams can calculate an emissions figure. Fewer can hand an assurance provider the meter reading, the invoice, the conversion factor, the version of that factor, and the name of the person who approved it, for every number in the filing. Assurance checks the chain of custody, and a chain of custody cannot be rebuilt in the last quarter of the year.

Governance claims become testable. “The Board oversees climate-related risks” has appeared in Japanese integrated reports for years. Under assurance 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 will have an uncomfortable first year.

Two reports, two numbers. Japan’s transition relief lets sustainability disclosures be filed after the financial statements for two years, with simultaneous filing afterwards. The integrated report keeps its own timeline and its own numbers. A company will then have an assured figure in the securities report and an unassured figure in the marketing document for the same year. Where the two differ, the assured figure is the one of record.

An Infrastructure Problem

The usual response is to add people: another sustainability manager, another consultant, a longer data-collection cycle.

The underlying problem is not effort. 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 survives disclosure. It does not survive an assurance provider whose first question is “where did this number come from.”

The cheaper path is to handle sustainability data the way finance already handles the ledger: one source of record, entries that carry their own provenance, controls that run through the year, outputs generated from the record rather than assembled beside it. Building that for a group with dozens of sites takes well over a year, which puts the start date in 2026.

Automated data capture, lineage tracking and machine-assisted mapping between frameworks are what make that record possible at group scale.

What Assured Data Does to Capital

Sustainability data across the region has been plentiful and hard to compare: different boundaries, different methodologies, different vintages, and until now no external check. Many investors have treated ESG disclosure as narrative for that reason.

From March 2028, companies representing 54.1% of Prime will publish emissions and governance information that an independent party has examined, on a common standard, inside a regulated filing. Data in that form can go into screens, models, covenants and diligence alongside the financials.

A company with a real operational story gains a claim the market can price. A company whose sustainability position is mostly narrative will have that show in the assured filing from March 2028 onward.

Before March 2027

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

Run a dry-run assurance this year. Ask an assurance provider, or the internal audit function, to test the current process against the published scope. Findings that arrive now can be fixed in systems instead of explained in footnotes.

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

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

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

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Japan has set the verification date a year after the disclosure date and named the claims that get checked first. Planning to March 2028 leaves two years to build the record. Planning to March 2027 leaves one.

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