Idemitsu said it and two other Japanese companies NX Nippon and Teikoku have found oil and gas off southern Vietnamese coasts.
The suspected oil and gas reserves are at the Dai Nga Well in Blocks 05-1b and 05-1c, about 300 km southeast of Ho Chi Minh City.
Idemitsu said it is assessing the reserves.
A wholly owned Idemitsu unit and JX Nippon Oil & Gas Exploration, part of JX Holdings, each holds a 35 percent stake in the blocks, while an Inpex Corp unit has the rest, Reuters reported.
The three firms entered into a production sharing contract with Petrovietnam in 2004.
This happens in the context of Japan's energy sector stepping up exploration activity in Southeast Asia.