Japan's pension fund is so big that a $13.5 billion investment barely makes the footnotes.

Yet there, in the back of the retirement manager's annual report, lies good news for the nation's so-called shame gauge. The $1.1 trillion Government Pension Investment Fund had 11 times as much money tracking the JPX-Nikkei Index 400 at the end of March as it did a year earlier. It's estimated to own more than half the assets following the stock measure.

After a tentative start, the retirement fund is becoming a powerful backer of the index created last year with an unusual mission: to change Japanese companies' behavior. The JPX-Nikkei 400, which picks the nation's most profitable firms in a bid to shame the others into improving performance, was set up partly to find stocks for GPIF to invest in as the fund shifts away from bonds.