When starting of a fuel gas consuming device configured to consume a fuel gas is instructed, an electric power is supplied to a fuel gas sensor that has a predetermined unstable period until an accuracy of fuel gas detection reaches a predetermined level after having been supplied with electric power, supply of the fuel gas from a fuel gas supply device to the fuel gas consuming device is started by opening an electric valve that is electrically opened and shut off. When a signal from the fuel gas sensor exhibits behavior of approaching a predetermined determination threshold value before the unstable period has elapsed, the electric valve is shut off. When the signal from the fuel gas sensor is less than a predetermined determination value in a predetermined period after the electric valve has been shut off, the electric valve is opened.