NOVELTY - The system (10) has coolant supply mechanism (16) to supply coolant to fuel cell stack (14), with coolant pipes (28,30) on front side in longitudinal direction of vehicle (12). The oxygen-containing gas supply mechanism (18) and fuel gas supply mechanism (20) supply oxygen-containing gas and fuel gas to stack, respectively. Pipe connects radiator (24) to stack in longitudinal direction of vehicle. Gas supply mechanism includes fuel gas pipe attached to rear of stack in longitudinal direction of vehicle, such that coolant and gas pipes are connected to stack ends.    USE - In-vehicle fuel cell system.    ADVANTAGE - The coolant and gas supply mechanisms can be respectively provided with coolant pipe and oxygen-containing gas and fuel gas pipes provided on the front and rear sides in traveling direction of vehicle relative to the fuel cell stack, so that the pipes extending from the radiator and fuel tank provided on the front and rear sides of the vehicle for supplying/discharging the coolant and fuel gas to/from the fuel cell stack are short. Thus the pressure losses in the pipes can be reduced and the pipe weight can be reduced. The piping structure can be simplified. Number of components is reduced at the end of the fuel cell stack in the stacking direction, thus the overall weight of the in-vehicle fuel cell system can be easily reduced. Dedicated mechanism for maintaining the temperature of the reactant gas auxiliary devices is not required, so that the temperature of the reactant gas auxiliary devices is suitably maintained with a simple and economical structure. Freezing of the fuel gas auxiliary devices is prevented when system is used in cold regions.    DESCRIPTION OF DRAWING(S) - The drawing shows a side view of the vehicle with fuel cell system.    In-vehicle fuel cell system (10)    Vehicle (12)    Fuel cell stack (14)    Coolant supply mechanism (16)    Oxygen-containing gas supply mechanisms (18)    Fuel gas supply mechanism (20)    Radiator (24)    Coolant pipes (28,30)