A fuel cell system comprising: a fuel cell 1 having a fuel gas passage 1D and an oxidizing gas passage 1E; a fuel gas feeder 2; an oxidizing gas feeder 3; a fuel gas exhaust passage 8; an oxidizing gas exhaust passage 9; a test gas feeder 20 configured to feed a test gas to either the fuel gas passage 1D or the oxidizing gas passage 1E; a flow rate detector 5 configured to detect the flow rate of the test gas; a first passage blocking device 4; and a controller 10, wherein said controller 10 controls said first passage blocking device 4 to block off the passage and controls the test gas feeder 2 to feed the test gas to said fuel cell 1, thereby obtaining a detected value from the flow rate detector 5 or an airtightness value that is numerical information into which the detected value is converted.