UPDATE 1-Activist investor ValueAct has $612 mln stake in Japan's Olympus

* ValueAct's stake worth $612 mln at Olympus' Thursday shareclose

* ValueAct says bought Olympus shares for investmentpurposes

* Says will make important proposals depending oncircumstances(Adds more details from the filing, Olympus comment)

TOKYO, May 31 (Reuters) - U.S. hedge fund ValueAct Capitalhas become a major shareholder in Japanese medical equipment andcamera maker Olympus Corp with a 5.04 percent stakethat is worth around $612 million at current shares prices.

The stake hike comes at a time when activist investment isgaining momentum in Japan, with Prime Minister Shinzo Abeadvocating shareholders raise issues and have constructivedialogue with firms to strengthen corporate governance.

The reason behind buying Olympus shares is pure investment,ValueAct said in a regulatory filing on Thursday, adding that itwould also give company management advice or make importantproposals depending on circumstances.

Olympus, in a statement, said it would "continue to hold anactive dialogue (with ValueAct) as one of many shareholders andfoster mutual understanding over our growth strategy".

An Olympus spokesman declined to comment any further whencontacted by Reuters.

Olympus shares closed at 3,860 yen on Thursday.

ValueAct's stake nearly matches the 5.05 percent stake ownedas of end-March by Sony Corp, which was Olympus'third-largest shareholder at the time.

The filing shows ValueAct has been gradually building up itsstake in Olympus over the past two months, reaching a mandatorypublic disclosure of ownership of over 5 percent.

Representatives of the fund did not immediately reply toemails seeking comment outside regular business hours.($1 = 108.7300 yen)(Reporting by Makiko Yamazaki and Ismail Shakil; Editing byHimani Sarkar)