Japanese prosecutors will open an investigation into possible insider trading involving shares of Wal-Mart's Japanese subsidiary Seiyu, the Nikkei business daily said on its website on Tuesday.

The trading is suspected to have occurred when Wal-Mart turned Seiyu into a fully owned unit in 2007, the Nikkei said on its website, citing unidentified sources.

Wal-Mart officials declined to comment on the report, repeating a statement made on Nov. 5 when Japan's Securities and Exchange Surveillance Commission began an investigation into possible insider trading, that it would cooperate with authorities.