Direct Answer
The TOPIX Mid400 is a rule-based measurement of Japanese mid-cap equity benchmark, maintained by JPX/Nikkei. Like every index it owns nothing: it is a number produced by applying a published weighting formula to a selected list of companies, revised on a schedule the provider sets.
What this index covers
The TOPIX Mid400 covers Japanese mid-cap equity benchmark, within the Japan market. JPX/Nikkei defines which securities are eligible, how they are selected from that pool, and how much each one contributes to the published number.
The name states a target of 400 companies. A target is not a guarantee of the exact count on any given day: corporate actions, dual share classes and pending additions all move the real number around it.
How constituents are weighted
Free float adjusted market capitalization weighted, using the same free float weight that TOPIX uses and the market cap weighting formula in the Index Calculation Guidebook. Price return and total return versions are calculated.
The weighting method is the single biggest driver of how an index behaves. Two indexes covering the same market can diverge for years purely because they weight differently, which is why comparing index levels without knowing the method tells you very little.
What qualifies for inclusion
The constituents of TOPIX 500 excluding those in TOPIX 100. The 400 names are the largest by market cap among the TOPIX Selected Issues and TOPIX 500 constituents as of the base date that rank in the top 1,000 by trading value and the top 600 by market cap, with any shortfall filled in trading value order from the top 1,000.
Eligibility rules are the part of an index most readers never look at and the part that decides what it can possibly measure. A screen that excludes a whole sector, or admits only one listing venue, shapes the result far more than any day of trading does.
When the membership is revised
Reviewed annually in October with a base date of the last business day of August, results published on the fifth business day of October and applied from the last business day of October. Between reviews a newly listed issue joins Mid400 only if it ranks in the top 400 by market cap and the top 1,000 by trading value, and delistings are not backfilled until the next October review.
Reconstitution is the one date an index changes shape by design rather than by price. Funds tracking it have to trade toward the new membership, which is why the schedule matters to anyone holding the tracker rather than only to the provider.
The rule that surprises people
Mid400 is the middle slice of nested indexes (Core30, then TOPIX 100, then TOPIX 500), so the number of constituents is a target of 400 after the October review and can drift between reviews through monthly inclusions and delistings.
References
- JPX/Nikkei: JPX Market Innovation and Research TSE Index Guidebook (TOPIX New Index Series, Size based TOPIX Sub Indices), November 2, 2026 edition published July 24, 2026, as published 2026. Every rule described on this page is taken from this document.
Current TOPIX Mid400 constituents
JPX/Nikkei publishes and licenses the TOPIX Mid400 constituent list. Swoopr does not republish it here. The provider's own page is the authoritative source for current membership and weights.
Related reading
- All stock market indexes: how indexes are built, weighted and revised.
- Asia-Pacific indexes: others covering the same region.
- Index concentration: measuring how much of an index sits in its largest holdings.
- Index rebalancing and inclusion effects: what happens to a stock when index membership changes.
Frequently Asked Questions
Who maintains the TOPIX Mid400?
JPX/Nikkei maintains the TOPIX Mid400 and publishes the methodology document that defines its eligibility and weighting rules.
What does the TOPIX Mid400 measure?
It measures Japanese mid-cap equity benchmark. The index is a calculated number applying a weighting formula to a selected list of companies, and it holds nothing itself.
Where can I see the current TOPIX Mid400 constituents?
JPX/Nikkei publishes the current constituent list. Swoopr does not republish it, because index providers license constituent data and Swoopr holds no such licence.