The director of Japan\'s crippled Fukushima Daiichi nuclear plant is stepping down due to illness, the facility\'s operator said Monday but it was not clear if his condition is radiation-related. Masao Yoshida, 56, who has been on site at the plant since Japan\'s March 11 earthquake and tsunami disaster, has been hospitalised for \"treatment of illness\", 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. \"He is hospitalised where he is able to take time in his convalescence.\" However, there were mixed messages from TEPCO, with senior official Junichi Matsumoto saying according to Jiji Press, \"We have heard from doctors that his condition is not related to radiation but it was not a definitive diagnosis.\" Another TEPCO spokeswoman Ai Tanaka told AFP: \"We have not yet heard from doctors about any causal relationship to radiation.\" The March disaster knocked out the atomic plant\'s cooling system and sent some of its reactors into meltdown, leaking radiation into the air, sea and food chain in the worst nuclear accident since the 1986 Chernobyl disaster. The plant 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. Yoshida, who assumed the post in June last year, said in a message to officials and workers at the plant, \"A condition was discovered during a regular medical check the other day.\" \"I had no choice but to be hospitalised at very short notice for treatment under doctors\' advice,\" said the message released by the operator. \"It breaks my heart to part with you, who have worked together since the earthquake disaster, in this way and I apologise from my heart for causing trouble to you,\" he said. \"I will focus on my treatment and stay strong so that I can come back to work with you as soon as possible.\" Yoshida is widely seen as a gutsy chief who continued injecting seawater into one of the troubled reactors at the early stages of the crisis, against the company\'s orders. He was reprimanded for the action which later proved to have been justified. The top government spokesman, Chief Cabinet Secretary Osamu Fujimura, told reporters: \"We will keep a close watch and see to it that this will have no adverse effect on the settlement of the nuclear accident.\" 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 endured a frightening ordeal in March. \"In the first week immediately after the accident I thought a few times \'I\'m going to die\',\" he said. And when a hydrogen explosion tore apart the buildings around reactors 1 and 3 in the days after the quake, he said: \"I thought it was all over.\" Yoshida said there were still hot spots of dangerously high radiation in the compound but that people could be reassured that the reactors were now stabilised. TEPCO told journalists on the day of the tour, a Saturday, that there were around 1,600 people at the plant, half of the usual weekday number, working to tame the reactors. The atomic crisis 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.