To return a shutoff valve to a valve opening state to surely supply fuel by immediately conducting the valve opening control when the pressure of gas fuel becomes designated pressure or less.

While retention control is conducted after the valve opening control, sometimes a shutoff valve is intercepted due to various causes such as instantaneous shutdown or drop of a current value. Even in the case where the shutoff valve is closed due to some reason during the operation of an engine 1, the shutoff valve is surely returned to the valve opening state so as to surely supply fuel to the engine 1. To be concrete, secondary or tertiary gas pressure are monitored by a second pressure sensor 9 and a third pressure sensor 14, and when the shutoff valve (P/R shutoff valve 8a and a delivery shutoff valve 13) is intercepted during the retention control so that the pressure of compressive natural gas is lowered, the valve opening control is conducted to open the shutoff valve and then the transition to the retention control occurs.