Japan does not have its own nuclear weapons. The Japanese government considered developing them in the past, but decided this would make Japan less secure. Japanese opinion polls consistently express strong public opposition to nuclear weapons. So do their elected representatives.
There is, however, a small group of non-elected Japanese bureaucrats with close ties to the U.S. defense establishment who insist U.S. nuclear weapons should be “the core of Japan’s security arrangements.” Wonks refer to this supposed core as “extended nuclear deterrence.” Journalists and politicians, especially in Japan, call it a “nuclear umbrella.”