The son of the founder of Idemitsu Kosan Co. has begun buying shares in rival refiner Showa Shell Sekiyu KK in a bid to block a proposed merger between the two companies.

Shosuke Idemitsu purchased 400,000 Showa Shell shares and may buy more until his namesake company gives up on the deal, according to a statement distributed to reporters in Tokyo on Wednesday.

Showa Shell rose as much as 12 percent to ¥1,014 the same day for its biggest intraday gain in more than a year, and closed 3.8 percent higher. Idemitsu fell 3.9 percent to ¥1,984.