A control unit performs control to previously set a transition determination filter value for determining a transition operation state of an engine, and calculate a transition determination intake pipe internal pressure from the transition determination filter value, an intake pipe internal pressure that was detected last time, and an intake pipe internal pressure that has been detected this time, and when a lapse time after start of the engine exceeds a predetermined value, calculate a transition determination intake pipe internal differential pressure from the intake pipe internal pressure that has been detected this time and the transition determination intake pipe internal pressure, set an intake pipe internal pressure filtering value based on the transition determination intake pipe internal differential pressure, and calculate an intake pipe internal pressure that is used for gas fuel injection control this time from the intake pipe internal pressure filtering value, the intake pipe internal pressure that has been detected this time, and an intake pipe internal pressure that was used for gas fuel injection control last time.