The new Japanese ambassador to China was hospitalised in Tokyo on Thursday, an official said, with media reporting he had collapsed unconscious.
Shinichi Nishimiya, who was officially appointed on Tuesday, collapsed on the street near his home in the fashionable Shibuya district of the Japanese capital, Jiji Press reported.
Kyodo News reported that the ambassador was unconscious.
An official at the foreign ministry confirmed Nishimiya, 60, had been taken ill.
"We learned that ambassador Nishimiya was hospitalised this morning for poor health, but don't have any more details," he told AFP.
The official added that he was not aware of any accident.
Jiji press, citing an unnamed government source, said foul play was not suspected.
The Sankei Shimbun said a passer-by who saw the ambassador stumble and fall on a Tokyo street made an emergency call to police.
He was clad in a business suit, the Sankei said, citing an unnamed police source.
Nishimiya, a career diplomat, replaces Uichiro Niwa at a time Japan and China are at loggerheads over a disputed island chain in the East China Sea.
Niwa, a former businessman, raised hackles at home earlier this year when he said a plan by Tokyo's nationalist governor to buy the islands could cause a crisis between China and Japan, jeopardising economic ties.
Nishimiya's appointment came on the day the government in Tokyo said it had finalised the purchase of three islands in a chain it administers under the name Senkaku, but which are claimed by China as Diaoyu.
The archipelago, which lies around 200 kilometres (125 miles) from Taiwan, sits near an important shipping lane and nearby waters are believed to harbour valuable mineral resources.