The announcement accompanied an admission by the president of Sumitomo, Kenji Miyahara, that the company first learned "some facts" on the activities of its former head of copper trading, Yasuo Hamanaka, from a Panorama programme broadcast by the BBC last week. Last week Mr Hamanaka pleaded guilty to fraud and forgery in the Tokyo District Court.
Mr Miyahara said the company's internal investigation would take "several more months", and had been delayed by the inadequacy of information in internal Sumitomo documents.
In a letter to the company's New York lawyers, Mr Akiyama said that Sumitomo "showed itself to be a model for international corporate behaviour" in its handling of the affair.
Japanese executives often respond to scandals by resigning, although in many cases it is little more than a symbolic gesture which still allows them to exercise power behind the scenes. Mr Akiyama has been appointed an "adviser". Asked what his new salary would be, a spokesman replied, "We are not obliged to disclose that."