To provide a mirror cycle type gas engine provided with a turbocharger detecting a boost pressure and a combustion chamber wall temperature, and in accordance with information of these detection values, properly controlling a fuel supply amount and the boost pressure.

This gas engine has a turbocharger 15 provided with an intake valve lift control device 30 changing open/close timing of an intake port 9 of an intake valve 11 and a motor generator provided in an exhaust pipe 31. A fuel flow amount supplied by a fuel supply means in response to a detection signal detected by each sensor respectively detecting a combustion chamber wall temperature of a main chamber 1, boost pressure of intake air supplied to the main chamber 1, engine output, and an ignition condition of fuel supplied to a sub-chamber 2, an operating condition of the motor generator of the turbocharger 15, and open/close timing of the intake valve 11 by the intake valve lift control device 30 can be controlled, in a controller.