An integrative control method and device for controlling gas engines is proposed which is improved load responsivity of the engine in transient operation. The control method comprises a speed control process for controlling engine rotation speed by controlling the fuel gas flow control valve based on deviation of actual engine rotation speed from a target command value of rotation speed, and an air fuel ratio control process for controlling air fuel ratio of fuel-air mixture by controlling throttle valve opening based on deviation of the actual mixture flow rate from the command value of mixture flow rate, whereby at least either fuel gas flow correction or fuel-air mixture flow correction is performed when time-series variation of input signals relating to performance change of the gas engine exceeds a reference range determined beforehand, the fuel gas flow correction being performed by correcting control variables of the fuel gas flow control valve in the speed control process, and the mixture flow correction being performed by correcting opening of the throttle valve in the air fuel ratio control process.