A control system of a gas engine for controlling a power output of the gas engine which uses a gas fuel as a main fuel, the control system comprising: a target value setting section for setting target value of the power output according to a source gas pressure of the gas fuel; a power output setting section for setting a set value of the power output based on the target value set by the target value setting section and a requested value of the power output; and a power output control section controlling the power output such that the power output reaches the set value set by the power output setting section; wherein the target value setting section includes: a primary setting section for setting a primary target value of the power output such that the primary target value decreases as the source gas pressure decreases, when the source gas pressure is less than a predetermined pressure value corresponding to a full load; and a secondary setting section for setting a secondary target value according to an allowable change rate, as the target value to be input to the power output setting section, when a change rate of the primary target value which is set by the primary setting section is greater than the allowable change rate; wherein the allowable change rate includes a first allowable change rate and a second allowable change rate which is greater than the first allowable change rate; and wherein the secondary setting section compares the change rate of the primary target value to the first allowable change rate, when the primary target value increases, and compares the change rate of the primary target value to the second allowable change rate, when the primary target value decreases. a target value setting section for setting target value of the power output according to a source gas pressure of the gas fuel; a power output setting section for setting a set value of the power output based on the target value set by the target value setting section and a requested value of the power output; and a power output control section controlling the power output such that the power output reaches the set value set by the power output setting section; wherein the target value setting section includes: a primary setting section for setting a primary target value of the power output such that the primary target value decreases as the source gas pressure decreases, when the source gas pressure is less than a predetermined pressure value corresponding to a full load; and a secondary setting section for setting a secondary target value according to an allowable change rate, as the target value to be input to the power output setting section, when a change rate of the primary target value which is set by the primary setting section is greater than the allowable change rate; a primary setting section for setting a primary target value of the power output such that the primary target value decreases as the source gas pressure decreases, when the source gas pressure is less than a predetermined pressure value corresponding to a full load; and a secondary setting section for setting a secondary target value according to an allowable change rate, as the target value to be input to the power output setting section, when a change rate of the primary target value which is set by the primary setting section is greater than the allowable change rate; wherein the allowable change rate includes a first allowable change rate and a second allowable change rate which is greater than the first allowable change rate; and wherein the secondary setting section compares the change rate of the primary target value to the first allowable change rate, when the primary target value increases, and compares the change rate of the primary target value to the second allowable change rate, when the primary target value decreases.