Fuel cell system

A fuel cell system, comprising:

• a fuel cell that generates power through an electrochemical reaction between hydrogen gas and oxygen gas, and discharges hydrogen off-gas comprising an unreacted portion of the hydrogen gas together with nitrogen gas and moisture;
• a hydrogen supply passage to supply the hydrogen gas from a hydrogen supply source to the fuel cell;
• a return passage to return the hydrogen off-gas to the hydrogen supply passage;
• a gas-liquid separator that is provided on the return passage, obtains dehumidified hydrogen gas by separating the moisture from the hydrogen off-gas, and stores the separated moisture;
• an exhaust valve to discharge the dehumidified hydrogen gas from the return passage;
• a pressure sensor to detect pressure in the return passage;
• and
• a control device that controls the exhaust valve,
• wherein no pump to pump out the dehumidified hydrogen gas is arranged on the return passage, and
• wherein the control device estimates a concentration of the nitrogen gas included in the dehumidified hydrogen gas based on a detection value of the pressure sensor, and controls the exhaust valve to open when the concentration of the nitrogen gas becomes not less than a predetermined value.