It is an object of the invention to achieve a stable idling revolution speed in a short time and to prevent easily the occurrence of abnormal combustion, misfire, or the like by effectively controlling a gas flow rate adjustment valve even in a case where a fuel gas which tends to fluctuate in fuel gas concentration, as represented by a coal mine gas, is used. The invention includes a revolution speed detection part 5 detecting the revolution speed of a gas engine 1, a gas concentration detection unit 7 detecting the gas concentration of a fuel gas, a gas flow rate adjustment valve 4 disposed in a fuel gas pipeline 3 supplying a fuel gas to the gas engine 1, a governor actuator 21 for operating the gas flow rate adjustment valve 4, and an operation amount computation part 15 computing the operation amount of the governor actuator 21 according to the gas concentration of the fuel gas detected by the gas concentration detection unit 7; and a governor controller 10 operating the governor actuator 21 based on the operation amount computed by the operation amount computation part 15 is further provided.