In a statement, the Tokyo Electric Power Company (TEPCO) said that a public declaration of the meltdown should have been done within days of the disaster.
Instead it denied that a meltdown was taking place for almost two months.
More than 160,000 people were evacuated from the area near the Fukushima meltdown, the worst nuclear accident since the 1986 Chernobyl disaster.
The company said internal regulations that a meltdown should be declared if damage to the reactor core exceeded 5% were ignored.
Damage to one of the reactor cores exceeded 50% within three days of the tsunami, but TEPCO did not acknowledge that a meltdown was taking place until months later. Read more on edition.cnn.