An object is to allow even a temporary increase in air pressure to be diagnosed as an abnormality. There is provided a fuel cell system including a fuel cell. The fuel cell system comprises an air system that is configured to supply the air to the fuel cell and discharge the air from the fuel cell; a pressure detector that is configured to detect an internal pressure in the air system; a pressure reduction controller that is configured to reduce the internal pressure in the air system to a target pressure reduction value, when the detected pressure becomes higher than a first abnormality detection value during a predetermined time period (S150 and S200 to S220); and a first determiner that is configured to determine that the air system has an abnormality upon determination that number of times when it is determined that the detected pressure becomes higher than the first abnormality detection value is equal to or higher than a predetermined number of times during the predetermined time period (S150, S180, S190 and S160).