A control system for controlling a power output of a gas engine of the present invention includes a target value setting section for setting as a target value a restricted power output which is less than a predetermined power output when a source gas pressure of a gas fuel is less than a predetermined value required to inject the gas fuel against an intake-air pressure according to the predetermined power output, a power output setting section for setting a set value of a power output based on the target value set by the target value setting section, and a power output control section for controlling the power output so that the power output reaches the set value set by the power output setting section.