The Japanese Foreign Ministry said Shinichi Nishimiya, 60, was taken to a hospital because of his "health condition," but did not elaborate, CNN reported.
Tokyo police said the diplomat was found collapsed, but did not provide details on who found him or where he was found.
Nishimiya was due to succeed the current Japanese ambassador in Beijing, Uichiro Niwa, in coming weeks. His appointment comes during a tense time diplomatically between Japan and China.
This week, the Japanese government bought a set of disputed islands in the East China Sea from a private owner, a move that angered China, which called the acquisition "illegal and invalid," CNN said.
China maintains its claim to the islands goes back hundreds of years. Japan, on the other hand, said it found nothing concerning Chinese control of the islands in an 1885 survey and has formally recognized them as Japanese sovereign territory in 1895. Japan sold the islands in 1932 to descendants of the original settlers.
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