When a stop trigger of a fuel cell system (100) is turned on, air humidified by a humidifier (3) which air having a humidity quantity lower than a humidity quantity at a normal operation is supplied to a fuel cell stack (11). Thereby, a takeout quantity Qm of a moisture generated in the fuel cell stack (1) is increased, then, a power generation of the fuel cell stack (1) is continued for a certain time Pg. Then, the power generation is stopped, and a cathode side of the fuel cell stack (1) is purged with the air for a certain time Pp.