To optimize a fuel supply quantity when an EGR rate and the like is changed by interposing an exhaust gas recirculating mixer mixing air and exhaust gas together and a fuel mixer mixing air and fuel gas together in an intake pipe in a fuel supply mechanism for a gas engine provided with an exhaust gas recirculating means.

An EGR mixer 2, a CO2 sensor 4, and a fuel mixer 3 are interposed in an intake pipe 1 of an engine E, and an EGR pipe 8 is connected to the EGR mixer 2 via an EGR quantity controller 6, while a fuel gas pipe 9 is connected to the fuel mixer 3 via a fuel controller 7. If an EGR quantity is changed during operation of the engine E, an EGR rate is detected by means of a fuel flow rate deciding means 11 on the basis of an output signal from the CO2 sensor 4. Then, on the basis of the detected EGR rate, a ratio of an actual air flow rate is computed, and then, a reduction ratio of the fuel gas flow rate is computed on the basis of the computed ratio. A signal complying with the computed reduction ratio is fed to a fuel controller driving unit 7a so as to drive the fuel controller 7, and consequently, the fuel flow rate can be corrected to a proper value.