An integrative control method and device for controlling gas engines is proposed which load responsivity of the engine is improved while maintaining air fuel ratio control and stable control is performed when fuel gas of different calorific value is used. 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 correction of theoretical air fuel ratio in accordance with exhaust temperature is performed in the air fuel ratio control process, the correction being done to correct the theoretical air fuel ratio used to calculate the command value of fuel-air mixture flow rate in accordance with exhaust temperature based on deviation of actual exhaust temperature from target exhaust temperature prescribed for various engine rotation speeds and load factors.