Tepco said workers at the site saw an oil slick floating on the sea at 8 a.m. Tuesday near the intakes of units 5 and 6.
The oil slick is believed to be 200 to 300 meters long.
The total amount of oil that has leaked is still unknown, and the utility plans to set up a boom to prevent the slick from spreading.
According to the utility, the two tanks, whose capacity is 960,000 liters each, were hit by the March 11 earthquake and tsunami and the tanks themselves or pipelines sustained damage, probably causing the oil spill.
Tepco spokesman Junichi Matsumoto said the utility believes the leak probably started on or shortly after March 11, noting the tsunami moved the tanks more than 10 meters to the north.
When the tsunami hit, a tanker was refilling the tanks and they were nearly full, Matsumoto said during a news conference at Tepco headquarters.
After the tsunami, workers visually checked the tanks from the outside, and did not conduct a detailed inspection, Tepco said.
Akio Koyama, a professor at Kyoto University's Research Reactor Institute and an expert on managing radioactive waste, said heavy oil floating on the sea is unlikely be contaminated with radioactive materials released from the crippled nuclear plant.
Radioactive materials such as ionic cesium usually dissolve in water. Water and oil are immiscible, so radioactive materials in the water rarely get absorbed by oil, Koyama said.
Radioactive materials in the air might stick to the oil, but the amount would be less compared with what's in the seawater, he added.