The Securities and Exchange Surveillance Commission has apparently begun investigations over possible insider trading in connection with the April reshuffle of the benchmark Nikkei Stock Average's components, industry sources said Monday.

Trading in almost all of the 30 shares excluded from the 225-issue index soared just one or two days before the announcement of the index's reshuffle.

The SESC has asked securities houses to submit trading reports, the sources said.

On April 15, Nihon Keizai Shimbun Inc., which controls and operates the Nikkei Stock Average on the Tokyo Stock Exchange, announced it would replace 30 of the issues on the benchmark index from April 24.

It was the first time in a decade that the newspaper company had replaced components of the Nikkei, saying the move was in line with new standards adopted to better reflect trends in the stock market.

Immediately after the announcement, the prices of the 30 stocks excluded from the index all declined sharply. Traders who sold them on credit before the announcement and later bought them back could have made huge profits.

On April 13, trading volume on the First Section of the TSE was only 5.2 percent higher than the day before, but that of the 30 issues shot up by 53.8 percent, with volume on Mitsubishi Steel Mfg., for instance, almost tripling.

Trading in the 30 stocks then jumped 50.7 percent on April 14, whereas the entire First Section saw a decrease of 4.7 percent, the sources said.

The 30 issues had comprised only 1 percent to 2 percent of total trading volume on the First Section from the beginning of April through April 12, but soared to 2.7 percent on April 13 and 4.2 percent the following day.

A surge in volume alone does not prove insider trading, but if this is shown to have taken place it could undermine the credibility of the Nikkei index, the sources said.