TOKYO, May 25 (Reuters) - Japanese equity fund Unison Capital on Monday 25 May raised its planned offer for shares in wig maker Aderans Holdings (8170.T) by 20 percent, making the bid at least 17.6 billion yen ($186 million).

Unison Capital has been locked in a battle with U.S. hedge fund Steel Partners for sharefolder votes at Aderans' annual general meeting scheduled for Thursday 28 May.

Steel Partners, Aderans' main shareholder with a 27 percent stake, had urged other shareholders of the loss-making company to reject Unison Capital's earlier offer as too low.

"The reason (for raising the planned offer) is to show our commitment to Aderans' management reform," said a spokeswoman for Unison Capital.

Aderans said last month it supported Unison Capital's bid for at least a 35 percent stake in Aderans for 1,000 yen per share. Unison Capital has said it would launch the bid if shareholders vote for the company's slate of board and other proposals.

Unison Capital's planned offer of 1,200 yen per share marks a 24 percent premium to Monday 25 May's closing price of 964 yen and puts the value of the 35 percent stake in Aderans at 17.6 billion yen.

A spokeswoman for Steel Partners said the company was studying Unison Capital's latest offer. ($1=94.57 Yen) (Reporting by Taiga Uranaka; Editing by Joseph Radford)