A gas engine comprising: a generator coupled to an output shaft of the engine; an intake passage to which a low-concentration methane gas derived from mine venting is supplied; a gas mixing unit (54a-54d) mixing a high-concentration methane gas to the low-concentration methane gas midway of the intake passage, so that a gas mixture of the low-concentration methane gas and the high-concentration methane gas is supplied to and burnt in a combustion chamber; a turbocharger provided in the intake passage upstream of the gas mixing unit (54a-54d); an air mixing part provided in the intake passage upstream of the turbocharger and mixing air with the low-concentration methane gas; a mixture ratio adjusting unit (26,28) adjusting a mixture ratio of the low-concentration methane gas and the air in the air mixing part; an intake controller controlling the mixture ratio adjusting unit (26,28) to keep the temperature or flow rate of intake gas flowing into the turbocharger in a predetermined range; a bypass passage arranged in parallel with a turbine of the turbocharger that is disposed in an exhaust passage, the bypass passage allowing part of exhaust gas to bypass the turbine; an exhaust gas flow rate control valve controlling flow rate of the exhaust gas in the bypass passage; a turbocharger controller controlling the exhaust gas flow rate control valve to control operation of the turbocharger, wherein the turbocharger controller controls flow rate of the intake gas passing through the turbocharger such that the gas mixture is supplied to the combustion chamber with a target excess air ratio; and wherein the intake controller includes an intake gas temperature control unit, and the intake gas temperature control unit controls the mixture ratio adjusting unit (26,28) to adjust the mixture ratio of the low-concentration methane gas and the air to keep the intake gas in the intake passage upstream of the turbocharger in a constant temperature range that allows stable control by the turbocharger controller to achieve the target excess air ratio. a generator coupled to an output shaft of the engine; an intake passage to which a low-concentration methane gas derived from mine venting is supplied; a gas mixing unit (54a-54d) mixing a high-concentration methane gas to the low-concentration methane gas midway of the intake passage, so that a gas mixture of the low-concentration methane gas and the high-concentration methane gas is supplied to and burnt in a combustion chamber; a turbocharger provided in the intake passage upstream of the gas mixing unit (54a-54d); an air mixing part provided in the intake passage upstream of the turbocharger and mixing air with the low-concentration methane gas; a mixture ratio adjusting unit (26,28) adjusting a mixture ratio of the low-concentration methane gas and the air in the air mixing part; an intake controller controlling the mixture ratio adjusting unit (26,28) to keep the temperature or flow rate of intake gas flowing into the turbocharger in a predetermined range; a bypass passage arranged in parallel with a turbine of the turbocharger that is disposed in an exhaust passage, the bypass passage allowing part of exhaust gas to bypass the turbine; an exhaust gas flow rate control valve controlling flow rate of the exhaust gas in the bypass passage; a turbocharger controller controlling the exhaust gas flow rate control valve to control operation of the turbocharger, wherein the turbocharger controller controls flow rate of the intake gas passing through the turbocharger such that the gas mixture is supplied to the combustion chamber with a target excess air ratio; and wherein the intake controller includes an intake gas temperature control unit, and the intake gas temperature control unit controls the mixture ratio adjusting unit (26,28) to adjust the mixture ratio of the low-concentration methane gas and the air to keep the intake gas in the intake passage upstream of the turbocharger in a constant temperature range that allows stable control by the turbocharger controller to achieve the target excess air ratio.