A U.S. activist investor has called on Japanese clothing company Sanyo Shokai Ltd. to sell itself, saying a new owner would help turn around the firm.

RMB Capital sent a letter in December to the company's board, urging it to seek a strategic buyer, according to Masakazu Hosomizu, a Chicago-based portfolio manager for the investment firm. Sanyo Shokai should seek a partner with more capital because its own efforts to revive the company haven't been successful and it doesn't have enough scale to reorganize its business on its own, he said.

RMB, which owns about 5 percent of Sanyo Shokai, decided to go public about the letter because it wasn't satisfied with the company's noncommittal response, Hosomizu said in an interview. A spokesperson for Sanyo Shokai wasn't immediately available to comment.