The woman, who worked in support functions at Fukushima mostly indoors, had received 17.55 millisieverts of radiation, Tepco said.
The limit for women is five millisieverts over three months, higher than for men because of risks if they become pregnant and because they have higher sensitivity to radiation and can be rendered sterile by lower doses.
"It was our error. We regret it," a Tepco official told reporters.
The woman, aged in her 50s, was pulled from work on 23 March along with 18 other female staff. Medical checkups found no health effects on her or 16 other women who have been tested so far.
Standard & Poor's has cut Japan's debt outlook in the wake of the disaster and estimated that reconstruction could cost up to 408 billion euros.
But the disaster should not affect the country's medium-term growth potential, the agency said.
On Tuesday more than 200 angry farmers demonstrated in Tokyo to demand compensation for products that were contaminated by radiation leaking from Fukushima.
The farmer's group marched to Tepco's headquarters with contaminated live cows and radiation-poisioned produce that the government has banned from being sold.
The group, making up some 5,440 households, estimates that they have already lost at least 15 million euros just in the month of March.
Tepco has announced that it would start compensation payments of 8,000 euros per household of those forced to evacuate.
Farmers are reported to have left behind nearly one million cows, pigs and chickens, some which have died already, and others who are quite ill.
But the cash is being distributed to people within 30 kilometres of Fukushima, and not to farmers further afield who have also been affected by the nuclear disaster.
The protesting farmers said that would not be enough. One farmer protester shouted that he was 70 years old and could not start over.
And many feared that consumers would refuse to buy products from Fukushima after the clean-up operation has finished.