The director of Japan's troubled Fukushima Daiichi nuclear power plant is stepping down because of illness, the plant's operator said Monday, without revealing whether his condition was radiation-related.

Masao Yoshida, 56, has been hospitalised for "treatment of illness" and will be relieved of his post as of Thursday, a spokeswoman for Tokyo Electric Power Co. (TEPCO) said.

"We cannot give you details of his illness because they are private matters," Chie Hosoda said, declining to say whether his illness was related to exposure to radiation.

"He is hospitalised where he is able to take time in his convalescence," she said.

Yoshida has been on site at the plant since it was hit on March 11 by a massive earthquake and tsunami, which knocked its cooling system out and left some of its reactors in meltdowns in the worst nuclear accident since the 1986 Chernobyl disaster.

It continues to leak radiation, although TEPCO and the government insist the reactors will all be brought to cold shutdown by the end of the year.

The Sankei Shimbun reported in its Internet edition that doctors had made no mention of a relationship between his illness and radiation.

Yoshida said in a message to officials and workers at the plant, "A condition was discovered during a regular medical check the other day," according to the Sankei.

"I had no choice but to be hospitalised at very short notice for treatment under doctors' advice."

Yoshida is being replaced by Takeshi Takahashi, who was in charge of nuclear power plant operations at TEPCO's head office in Tokyo, some 220 kilometres (140 miles) southwest of the Fukushima plant.

Yoshida told reporters on November 12 when the plant allowed a group of journalists to visit there for the first time that he had a very frightening time in March.

"In the first week immediately after the accident I thought a few times 'I'm going to die'," he said.

And referring to when a hydrogen explosion tore apart the buildings around rectors 1 and 3, he said: "I thought it was all over."

Yoshida also said there were still spots of dangerously high radiation in the compound but he wanted residents to feel relieved as reactors are now stabilised.

Tepco told journalists on the day, a Saturday, that there were around 1,600 people at the plant, half of the weekday number, working to tame the reactors.

The atomic crisis, which has seen radiation leaking into the air, sea and food chain, has not in itself claimed any lives but has badly dented the reputation of nuclear power, a key source of energy in resource-poor Japan.

Thousands of people remain evacuated from a large area around the plant, with no indication when the many who left homes and farms in the shadow of the reactors will be able to return.