To obtain a gas sterilization apparatus which continues a sterilization process even when air leakage occurs during the sterilization process.

The gas sterilization apparatus for supplying sterilization gas to the inside of a sterilization tank 1 for performing a sterilization treatment on an article to be sterilized, is equipped with depressurizing means 13 for reducing a pressure in the sterilization tank 1, a pressure sensor 16 for measuring a pressure inside the sterilization tank 1, and a controller 18. In the sterilization process, the depressurizing means 13 is controlled so as to keep the inside of the sterilization tank 1 at a prescribed pressure or lower based on a detected value by the pressure sensor 16, while a concentration of the sterilization gas is calculated and the sterilization time is controlled to be extended in accordance with the calculated concentration of the sterilization gas when the concentration of the sterilization gas is a prescribed concentration or lower.