A gas engine has a starting operation mode in which fuel gas flow is controlled based on actual mixture flow rate and adequate excess air ratio prescribed for each of detected values of operating conditions of the engine and then mixture flow is controlled so that engine rotation speed approaches a target rotation speed, and a normal operation mode in which fuel gas flow is controlled so that engine rotation speed approaches the target rotation speed and then mixture flow rate is controlled so that air fuel ratio of the mixture coincides with an adequate value prescribed for each of detected values of operating conditions of the gas engine with the fuel gas flow flowing at the commanded fuel gas flow rate, and a switchover rotation speed is predetermined so that operation mode is switched from starting mode to normal mode when engine speed reaches the switchover rotation speed.