To keep an engine in a well operated condition even when using non-thermally-conditioned gas as combustion gas while simplifying the construction of a fuel supply passage.

This gas engine driving mechanism comprises a fuel supply pipe 32 connected at its downstream side to a throttle portion 36 for supplying mixed gas of air and fuel gas to a gas engine 30, a fuel control valve 35 provided on the way of the fuel supply pipe, a target opening setting means 57 for setting the target opening of the fuel control valve, and a control device 13 for controlling the valve opening of the fuel control valve to be the target opening set by the target opening setting means. The control device 13 has a target opening correcting means 75 for calculating a revolution variation average value in accordance with revolution data detected by a revolution detecting means 72 after starting the engine and for correcting the target opening of the fuel control valve in accordance with the calculation result.