A zero-governor fitting structure for a gas engine, configured in such a manner that the lower half of a fuel tank for a gasoline engine is utilized as a support box for supporting the zero-governor in which a gas supply inlet pipe and a zero governor can be reliably mounted with vibration and heating of the zero-governor avoided. A zero-governor mounting structure for a gas engine equipped with a zero-governor is provided with a support box fixed to the upper part of the gas engine. The support box is formed as a box having two depths. The zero-governor is supported, through a vibration insulation gasket, on the bottom plate of the shallower depth space of the support box. A gas supply inlet pipe and an elbow joint which is connected to the gas supply inlet pipe and can be bent the direction of the gas flow at a right angle toward the zero-governor are received in the deeper depth space, and the elbow joint is connected to the bottom inlet of the zero-governor.