The ruling bloc was sailing to victory in three of the four Diet by-elections held Sunday, while the Democratic Party of Japan, the largest opposition force, was assured of winning in the House of Representatives by-election in Tokyo, early returns showed.

Yoko Komiyama, 54, the official candidate of the DPJ, was assured victory over Liberal Democratic Party candidate Michio Ochi, 74, in the race for the Lower House's Tokyo No. 6 constituency.

During the campaign, run in the heavily populated Setagaya Ward, Ochi emphasized his expertise in economic policies, having been a minister in charge of the government's financial policy board.