An airline spokesperson said a passenger suffered heart problem and was admitted to a local hospital.
"Air India Flt (sic) AI 173 Delhi to San Francisco diverted to SPK due to a medical emergency," the airline posted on Twitter.
There were 215 passengers on board the aircraft, the airline official said.
The plane took off from New Delhi's Indira Gandhi International Airport here at 4.30 am, and had flown past Japan when it had to turn back towards Sapporo, capital of the northern Japanese island of Hokkaido.
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