The former chief of Japan's crippled nuclear plant, who left the job last week, has cancer of the esophagus, his employer said Friday, adding it was unlikely to be linked to radiation exposure.

Masao Yoshida, 56, had been based at the plant since Japan's March 11 earthquake and tsunami disaster sparked reactor meltdowns, until he was suddenly hospitalised in late November.

The plant operator, Tokyo Electric Power Co. (TEPCO), did not at the time specify the reason for his being taken to hospital, amid speculation it could be related to radiation exposure.

But during a visit to the plant on Friday, "Yoshida himself disclosed he had esophagus cancer" as he spoke to workers there, according to TEPCO spokeswoman Ai Tanaka.

"He had been worried about media speculation over his illness... He wanted to concentrate on his treatment quietly but decided to disclose what it is" to stem the speculation, she said.

TEPCO believes "it is extremely unlikely that his disease was caused by radiation exposure," she said, citing the advice of a doctor at the National Institute of Radiological Sciences.

If radiation exposure were to cause cancer of the esophagus, it would take at least five years and normally 10 years to develop, she quoted the doctor as saying.

Yoshida's cumulative radiation exposure was 70 millisieverts since March 11, according to TEPCO -- a level lower than the boosted threshold for emergency workers.

The government set 100 millisieverts as the limit for workers involved in emergency operations at standard Japanese nuclear plants, but raised the threshold to 250 for the ongoing crisis at the Fukushima No. 1 plant.

The limit has been put back to 100 millisieverts for those who joined Fukushima operations from November 1.

TEPCO refused to give further details on Yoshida's illness, including whether he has undergone an operation or what treatment he was receiving.

The March disaster knocked out the atomic plant's cooling system and sent some of its reactors into meltdown, spewing radiation into the air, sea and food chain in the worst nuclear accident since the 1986 Chernobyl disaster.

No deaths have been directly attributed to the accident, but it has displaced tens of thousands of people and left large areas of land uninhabitable, possibly for decades.