A fuel cell system of the present invention includes: a fuel cell (11) configured to generate electric power using a fuel gas and an oxidizing gas; an anode off gas channel (34) through which an anode off gas discharged from an anode (2a) of the fuel cell (11) flows; a gas-liquid separator (10) disposed on the anode off gas channel (34) to separate moisture from the anode off gas, and including a water reservoir (18) configured to store the separated moisture as water; a temperature detector (28) configured to detect a temperature of the water reservoir (18); and an operation allowing device (52) configured not to allow an operation of the fuel cell system in a case where the temperature detected by the temperature detector (28) is equal to or higher than a first threshold that is higher than a standard ambient temperature.