Tokyo Electric, know as TEPCO, said the cooling facility restarted at 12:49 p.m. (0349 GMT) and was expected to lower temperatures which had risen at the reactor and fuel pool after the cooling system stopped working late on Saturday.
The temperature at the nuclear reactor at the No.5 unit was up to 87 degrees Celsius (188 degrees Fahrenheit) at 8:00 a.m. (2300 GMT) from about 68 degrees before the cooling system stopped operating, the official said.
The Fukushima Daiichi plant's No. 1 to 4 reactors were severely damaged after the March 11 earthquake and tsunami, while damages at the No. 5 and No. 6 reactors were less severe