"I understand that the bombing ended the war, and I think that it couldn't be helped," Kyodo News agency quoted Kyuma as saying in a speech at a university in Chiba, just east of Tokyo.
Kyuma's remarks drew immediate criticism from Japanese atomic bomb survivors.
"The U.S. justifies the bombings saying they saved many American lives," said Nobuo Miyake, 78, director-general of a group of victims living in Tokyo. "It's outrageous for a Japanese politician to voice such thinking. Japan is a victim."
Defense Ministry officials were not immediately available for comment Saturday.
On Aug. 6, 1945, the U.S. dropped a bomb nicknamed "Little Boy" on Hiroshima, killing at least 140,000 people in the world's first atomic bomb attack. Three days later, it dropped another atomic bomb, "Fat Man," on Nagasaki. City officials say about 74,000 died.
Japan, which had attacked the United States at Pearl Harbor, surrendered on Aug. 15, 1945.
Bombing survivors have developed various illnesses from radiation exposure, including cancer and liver diseases.
Kyuma, who is from Nagasaki, said the bombing caused great suffering in the city, but he does not resent the U.S. because it prevented the Soviet Union from entering the war with Japan, Kyodo said.