A gas engine drive system comprising: a gas engine including a combustion chamber formed by a cylinder and a piston; a turbocharger including a compressor connected to the combustion chamber via an intake passage and a turbine connected to the combustion chamber via an exhaust passage; a fuel injection system that injects a fuel gas into intake air that is supplied from the compressor to the combustion chamber via the intake passage; a pressure detector that detects a charge air pressure that is a pressure of the intake air; and a controller that controls the fuel injection system, the controller calculating a target injection amount of the fuel gas in accordance with a required output, wherein the controller: when the required output decreases, determines a lean limit of the charge air pressure based on the target injection amount corresponding to the required output that has decreased; if the charge air pressure detected by the pressure detector is lower than or equal to the lean limit, decreases a fuel injection amount to the target injection amount; and if the charge air pressure detected by the pressure detector is higher than the lean limit, brings the fuel injection amount to zero, and then when the charge air pressure detected by the pressure detector becomes lower than or equal to the lean limit, increases the fuel injection amount to the target injection amount. a gas engine including a combustion chamber formed by a cylinder and a piston; a turbocharger including a compressor connected to the combustion chamber via an intake passage and a turbine connected to the combustion chamber via an exhaust passage; a fuel injection system that injects a fuel gas into intake air that is supplied from the compressor to the combustion chamber via the intake passage; a pressure detector that detects a charge air pressure that is a pressure of the intake air; and a controller that controls the fuel injection system, the controller calculating a target injection amount of the fuel gas in accordance with a required output, wherein the controller: when the required output decreases, determines a lean limit of the charge air pressure based on the target injection amount corresponding to the required output that has decreased; if the charge air pressure detected by the pressure detector is lower than or equal to the lean limit, decreases a fuel injection amount to the target injection amount; and if the charge air pressure detected by the pressure detector is higher than the lean limit, brings the fuel injection amount to zero, and then when the charge air pressure detected by the pressure detector becomes lower than or equal to the lean limit, increases the fuel injection amount to the target injection amount. when the required output decreases, determines a lean limit of the charge air pressure based on the target injection amount corresponding to the required output that has decreased; if the charge air pressure detected by the pressure detector is lower than or equal to the lean limit, decreases a fuel injection amount to the target injection amount; and if the charge air pressure detected by the pressure detector is higher than the lean limit, brings the fuel injection amount to zero, and then when the charge air pressure detected by the pressure detector becomes lower than or equal to the lean limit, increases the fuel injection amount to the target injection amount.