Providing a gas engine in which the drain oil in the blow-by gas as a lubricant is mixed into the air fuel-gas mixture, the lubricant actively lubricating the bearing surfaces of the wear parts such as the valve seats and valve guides as to the intake-air valve and the exhaust gas valve, so that the lubricating device improves the engine lubricating conditions. A lubrication device for a gas engine thereby a valve gear room 6 is formed on a cylinder head, including: a blow-by gas discharge pipe 3 through which the blow-by gas flowing into the valve gear room 6 is discharged; whereby, an oil mist separator 4 is arranged at the downstream side of the blow-by gas discharge pipe, the oil mist separator separating the drain oil and the air in the blow-by gas; the outlet regarding the drain oil from the oil mist separator communicates with an oil discharge device 1 through an oil pipe 3s, the an oil discharge device being arranged at an upstream side of a gas mixer 110 mixing the fuel gas with the intake air for the engine, the upstream being in relation to the intake air; the outlet regarding the air from the oil mist separator communicates with an air inlet of the air cleaner 25 for the engine intake air, through a breathing pipe 5.