The downgrade brings Moody’s rating for Japan in line with Standard & Poor’s, which lowered the country’s grade by one notch to AA-minus in January, the fourth highest on its scale. Moody’s had put Japan on review for a downgrade in May.

The action comes after a round of downgrades by major ratings agencies of sovereign debt, and amid concern that the debt crisis in Europe could escalate. On Aug. 5, S.& P. cut the sovereign debt rating of the United States for the first time in the country’s history.

Markets in Tokyo largely shrugged off the downgrade, the latest in a line of many.

Trust in Japanese government debt “remains unwavering,” Japan’s finance minister, Yoshihiko Noda, told reporters after the downgrade.

Still, the move, a week before the country’s ruling party is to select a new prime minister, could put additional pressure on the incoming administration to balance budgets. The government financing of the recovery from the March 11 earthquake, tsunami and subsequent nuclear crisis is expected to reach as high as 10 trillion yen ($130 billion).

Even before the disasters, Japan’s debt was expected to soar to almost 220 percent of its gross domestic product next year, according to the Organization for Economic Cooperation and Development, which would rank it as the largest debt-to-G.D.P. ratio in the world. Japan, however, has long been able to borrow at low nominal rates because of unwavering appetite by domestic investors for government debt.