The Japanese Diet approved legislation on April 27 that will pave the way for Japan to join the International Criminal Court. Final accession to the Rome Statute is expected in October.
Barring an earlier entrant, Japan will become the 105th state to join the ICC. It will also become the largest financial contributor to the Court with a share of the ICC's budget that is expected to be approximately 16 percent.
For more on Japan's accession, see this Amnesty International press release, this Reuters news story, and two posts--here and here--by Kevin Jon Heller at Opinio Juris. The ICC's web site is located here.