A fuel cell system of the present invention is a fuel cell system including a fuel cell and includes a medium circulation passage, a heat medium tank, a first circulator, a recovered water tank, a water circulation passage, a second circulator, a water purifier, a temperature detector, and a controller. The heat medium circulation passage and the water circulation passage are configured so as to realize heat exchange between a heat medium and water. The controller executes a circulation operation in which when the temperature detector detects a temperature lower than a first temperature capable of sterilizing microorganisms, the second circulator is caused to operate such that the temperature detected by the temperature detector becomes the first temperature or higher. The controller forbids the operation of the second circulator when the temperature detector detects a temperature equal to or higher than a second temperature that is lower than a temperature at which heat deterioration of the water purifier occurs.