Tokio Marine Holdings Inc. said Wednesday it has agreed to buy U.S. insurer Philadelphia Consolidated Holding Corp. for $4.7 billion (about ¥500 billion) to expand in the world's biggest insurance market.

Tokio Marine, Japan's largest insurance group, said it will pay $61.50 per share for Philadelphia.

That's 73 percent more than the Pennsylvania-based company's closing price of $35.55 on Tuesday. The deal would reportedly be the biggest acquisition by a Japanese insurer.