Deterioration diagnosis of fuel cell seal member

A fuel cell stack (1) housed in a case (2) generates power according to supply of hydrogen gas to a hydrogen gas passage (111) formed inside the case. The hydrogen gas passage (111) is sealed by a seal member (107). The case (2) is ventilated by a ventilation device (7). A sensor (8) detects the hydrogen concentration of gas which a ventilation device (7) discharges from the case (2). The controller (10) calculates a variation rate of the hydrogen concentration after supply of hydrogen gas to the hydrogen gas passage (111) starts (S2), and correctly diagnoses the deterioration state of the seal member (107) from the variation rate (S7, S71, S72).