NOVELTY - The system (10) has a scavenging gas supply apparatus (18) for supplying air as a scavenging gas to a fuel gas flow field (36). A control device i.e. controller (20), has a gas concentration detection unit for detecting gas concentration in a fuel cell (22) after an operation of the fuel cell is stopped. A scavenging control unit (74) starts scavenging in the fuel gas flow field by the scavenging gas supply apparatus when a fuel gas i.e. hydrogen gas, remaining in the fuel gas flow field is determined to be replaced with the air based on the detected gas concentration.    USE - Fuel cell system for a motor vehicle.    ADVANTAGE - The system does not perform scavenging in the field using air, as the gas in the field is not replaced with an oxygen-containing gas, if an operation of the cell is started within a short period of time after stopping the operation, thus reducing the frequency of performing scavenging in the field, and hence effectively suppressing or preventing degradation of the anode by scavenging using the air at the time of starting the operation of the cell. The system reduces the frequency of using a compressor for scavenging in the field by using the air so as to suppress generation of noise from the compressor, thus preventing energy efficiency from being lowered. The system cools water remaining in the field in a period after stopping the operation until scavenging in the field is completed by using the air, so that water condensation easily occurs, thus reliably discharging the condensed water remaining in the field at the time of scavenging in the field by using the air, and hence effectively improving performance of starting operation of the system at a low temperature.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a scavenging method for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a fuel cell system.    Fuel cell system (10)    Scavenging gas supply apparatus (18)    Controller (20)    Fuel cell (22)    Fuel gas flow field (36)    Scavenging control unit (74)