Idemitsu Kosan Co (5019.T: Quote, Profile, Research, Stock Buzz) said on Tuesday it plans to invest as much as 600 billion yen ($6.2 billion) over the next three years, mostly on exploration and refining overseas.

The amount is nearly double the company's investment of 344 billion yen made over the past four years ended in March.

Japan's third-largest refiner said about 45 percent of the investment would be targeted at energy exploration and development while another 30 percent would be spent on oil refining, including its projects in Vietnam and Qatar.

Idemitsu, which has a combined crude refining capacity of 640,000 barrels per day (bpd) at its four refineries in Japan, has been curbing crude runs since August 2008 in the face of weak domestic demand.

Japan's oil use has declined sharply as its worst recession since World War 2 stifles industrial activity, adding to already weakening demand as the population ages and users shift towards greener energy sources.

($1=96.33 Yen) (Reporting by Osamu Tsukimori and James Topham)