Japanese refiners Idemitsu Kosan Co. Ltd. and Showa Shell Sekiyu K.K. will combine management of their key businesses, an Idemitsu spokesman has said, pursuing a merger bitterly opposed by a core investor — Idemitsu's founding family.

Japan's second- and fourth-biggest refiners will form an office next spring with a staff of about 300 to manage crude oil purchases, refining and sales, seeking to cut costs by ¥30 billion ($266 million) over three years, Idemitsu spokesman Akitaka Yoshino said Tuesday.

The cost-cutting target was raised from a forecast of at least ¥25 billion when the companies said they would start integrating operations. A spokeswoman at Showa Shell confirmed the plans.