NOVELTY - A fuel cell system (1) comprises an operation controller, a checking unit that determines whether or not fuel cell (10) has been started up in the low-temperature operation mode, a scavenging unit (40) that supplies a scavenging gas to reactive gas passage, an instructing unit that provides an instruction for a shutoff of electricity generation of the fuel cell, a temperature sensor (25) for detecting a temperature of the fuel cell, a humidifier (33) for humidifying the scavenging gas, and a humidification controller that controls an amount of humidification.    USE - The fuel cell system, e.g. polymer electrolyte fuel cell system, is used for a fuel cell automobile.    ADVANTAGE - The system facilitates proper shut-off operation. It is able to provide proper generation of electricity at a subsequent startup.    DETAILED DESCRIPTION - A fuel cell system comprising:    (1) a fuel cell having a reactive gas passage, the fuel cell generating electricity with a reactive gas supplied to the reactive gas passage;    (2) an operation controller that controls operation of the fuel cell at startup by selecting one of a normal operation mode and a low-temperature operation mode to expedite a startup of the fuel cell;    (3) a checking unit that determines whether or not the fuel cell has been started up in the low-temperature operation mode;    (4) a scavenging unit that supplies a scavenging gas to the reactive gas passage to scavenge the reactive gas passage;    (5) an instructing unit that provides an instruction for a shutoff of electricity generation of the fuel cell, where when the instructing unit transmits the instruction, the scavenging unit scavenges the reactive gas passage;    (6) a temperature sensor for detecting a temperature of the fuel cell;    (7) a humidifier for humidifying the scavenging gas to be supplied to the fuel cell; and    (8) a humidification controller that controls an amount of humidification provided by the humidifier.    When the scavenging unit conducts scavenging based on the checking unit determining that the fuel cell has been started in the low-temperature operation mode in response to an instruction for a shutoff of electricity generation, and an actual temperature of the fuel cell detected by the temperature sensor is higher than a predetermined temperature, the humidification controller controls an amount of humidification according to the actual temperature of the fuel cell. An INDEPENDENT CLAIM is included for a method for shutting off the fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram showing architecture of a fuel cell system.    Fuel cell system (1)    Fuel cell (10)    Temperature sensor (25)    Humidifier (33)    Scavenging unit (40)