The fuel cell system is provided with a radiator, a first flow passage in which refrigerant flows from a fuel cell stack towards a radiator, a second flow passage in which refrigerant flows from the radiator towards a fuel cell stack, a bypass flow passage that connects the radiator with a position in the second flow passage at which refrigerant flows into the fuel cell stack, and a control part. In the bypass flow passage, an on-off valve and a reserve tank are provided from the upstream side. The control part feeds refrigerant to the fuel cell stack by opening the on-off valve and allowing refrigerant previously stored in the reserve tank to join the second flow passage in a case where refrigerant temperature detected by an inflowing refrigerant temperature detection part exceeds previously set base temperature.