NOVELTY - The fuel cell system (1) has a thermostat valve (11) set so that coolant flows through a bypass circuit (10) when temperature of coolant detected by a temperature sensor (12) is below a first threshold value, and a drive mode judging unit (21) that selects the drive mode of first and second cooling water pump (5,9) based on detected temperature. The drive mode judging unit selects first drive mode that intermittently operates the first cooling water pump when detected temperature is below a second temperature threshold value set lower than the first temperature threshold value.    USE - Fuel cell system for fuel cell vehicle.    ADVANTAGE - Suppresses power consumption of the auxiliary machine since intermittent operation of single unit of first cooling water pump is performed by preventing excessive water content of membrane electrode structure and setting appropriate moisture content of membrane electrode structure. Prevents overheating of the membrane electrode structure since drive mode is terminated appropriately and switched to the following drive mode even if the temperature of fuel cell has not reached the temperature threshold value. Promotes warming-up when electric power generation stability of fuel cell is low since bypass circulation path is made to flow through the coolant intermittently when temperature of fuel cell is below the second temperature threshold value. The water content in membrane electrode is prevented from becoming excessive since flow volume is decreased in low-temperature area and temperature rising of membrane electrode structure is accelerated.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a starting method of the fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the fuel cell system. (Drawing includes non-English language text).    Fuel cell system (1)    Cooling water pumps (5,9)    Bypass circuit (10)    Thermostat valve (11)    Temperature sensor (12)    Drive mode judging unit (21)