A fuel cell system includes: an air compressor that sends out oxidant gas to a fuel cell stack, including a motor, a housing, and an impeller; an oxidant gas supply and discharge passage; a pressure-regulating valve; and a controller. The controller executes at least one of a first process and a second process, the first process being executed for increasing a speed of the impeller after decreasing an opening degree of the pressure-regulating valve in a first operation where both decreasing the opening degree of the pressure-regulating valve and increasing the speed of the impeller are performed, the second process being executed for increasing the opening degree of the pressure-regulating valve after decreasing the speed in a second operation where both increasing the opening degree of the pressure-regulating valve and decreasing the speed are performed.