The President of Tokyo Electric Power Company (Tepco), Masataka Shimizu, has been taken to hospital in Tokyo.
Mr Shimizu was admitted after suffering from dizziness and high blood pressure, according to the company.
He has not been interviewed since 13 March, sparking speculation about his health. Tepco shares fell 18%.
Tepco has been struggling to avert disaster at its Fukushima Daiichi nuclear plant, which suffered damage in this month's earthquake and tsunami.
The company's chairman Tsunehisa Katsumata has taken over at the helm.
Tepco has now said, for the first time publicly, that at least four of the plant's six reactors will have to be decommissioned once the crisis subsides.
"After pouring seawater on them... I believe we cannot use them anymore," said Mr Katsumata.
The Japanese government has been saying since 20 March that the entire plant must be scrapped.
The latest setback for Tepco comes amid reports that radiation leaking into the seawater from its plant has reached its highest level yet.
Nuclear safety officials said on Wednesday that radioactive iodine in seawater outside the plant was 3,355 times more than the normal, a sign that contaminated water was leaking into the ocean.
As the crisis continues, shares in the company tumbled with investors also growing increasingly concerned that the company will be nationalised.