TOKYO, Jan 29 (Reuters) - Japanese refiner Idemitsu Kosan Co said on Friday it had extended its public tender offer for a smaller rival, Toa Oil Co, in which a U.S. investment fund has been building up a stake. In a statement to the Tokyo Stock Exchange, Idemitsu said the tender would run until Feb. 15, as Toa Oil had revised its earnings forecast. Idemitsu launched the tender in December to buy all the shares of Toa Oil at 2,450 yen each in a deal worth 15 billion yen ($143 million). Toa shares have since surged to trade above Idemitsu’s offer. By Thursday, asset manager Cornwall Capital Management had boosted its stake to 22.98%, a public filing showed, amassing a stake sufficient to block a bid. Idemitsu, which already owns 50.12% of Toa, aims to acquire a further stake of at least 16.55% to succeed in its bid. Shares of Toa Oil closed at 2,805 yen on Friday. ($1=104.5500 yen) (Reporting by Junko Fujita; Editing by Clarence Fernandez)