TOKYO—Japan moved to secure its role in the global, civil nuclear sector Tuesday by setting up a government-backed organization to promote Japanese technology and fight off competition for multibillion-dollar contracts from rivals such as South Korea.

Six companies— Tokyo Electric Power Co. , Chubu Electric Power Co. , Kansai Electric Power Co. , Toshiba Corp. , Hitachi Ltd. , and Mitsubishi Heavy Industries Ltd. —said Tuesday that they have set up a preparatory organization to sell nuclear-power-plant construction and operation technologies overseas.

"This is the first, important step toward promoting Japan's nuclear-power technologies in emerging countries by private- and public-sector cooperation," said Masayuki Naoshima, Japan's Minister of Economy, Trade and Industry, following the joint announcement.

The Japanese government hasn't disclosed the value of its planned investment in the group.