In a fuel cell system (100), the amount of fuel gas supplied through an fuel gas supply passage (24) and the amount of fuel gas consumed by a fuel cell (10) are detected. A difference between the amount of supplied fuel gas and the amount of consumed fuel gas is calculated by subtracting the amount of consumed fuel gas from the amount of supplied fuel gas. The pressure of the fuel gas in a leak detection passage (Cx), the change in the pressure of the detected fuel gas can be detected, and the change in the amount of fuel gas in the gas leak detection passage (Cx) due to the change in the pressure of the fuel gas can thus be detected. The difference is corrected by subtracting the change in the amount of fuel gas in the gas leak detection passage from the difference. When the corrected difference is equal to or greater than a predetermined value, a fuel gas leak is detected in the leak detection passage (Cx).