A gas engine comprising: a mixer mixing air and a fuel to generate an air-fuel mixture; a cylinder head having an intake port sucking the air-fuel mixture from the mixer; an intake throttle valve capable of adjusting a flow rate of the air-fuel mixture flowing from the mixer to the intake port; a torque calculating means for calculating a torque of an output shaft; an excess ratio identifying means for identifying an air excess ratio of the air-fuel mixture; an excess ratio adjusting means (21, 22) for adjusting the air excess ratio; a throttle opening degree control means for controlling an opening degree of the intake throttle valve based on a signal indicative of the torque from the torque calculating means; and an excess ratio control means for controlling the excess ratio adjusting means (21, 22) such that the air excess ratio falls within a stoichiometric range, wherein while the excess ratio control means provides the control such that the air excess ratio falls within the stoichiometric range, if the throttle opening degree control means determines that a value of the torque is smaller than a first predetermined value, the throttle opening degree control means provides a first control of controlling the opening degree of the intake throttle valve such that the output shaft rotates at a constant first rotation number and, if the throttle opening degree control means determines that the value of the torque is larger than the first predetermined value, the throttle opening degree control means provides a second control of controlling the opening degree of the intake throttle valve such that the rotation number of the output shaft increases in accordance with a linear function as the value of the torque becomes larger than the first predetermined value. a mixer mixing air and a fuel to generate an air-fuel mixture; a cylinder head having an intake port sucking the air-fuel mixture from the mixer; an intake throttle valve capable of adjusting a flow rate of the air-fuel mixture flowing from the mixer to the intake port; a torque calculating means for calculating a torque of an output shaft; an excess ratio identifying means for identifying an air excess ratio of the air-fuel mixture; an excess ratio adjusting means (21, 22) for adjusting the air excess ratio; a throttle opening degree control means for controlling an opening degree of the intake throttle valve based on a signal indicative of the torque from the torque calculating means; and an excess ratio control means for controlling the excess ratio adjusting means (21, 22) such that the air excess ratio falls within a stoichiometric range, wherein while the excess ratio control means provides the control such that the air excess ratio falls within the stoichiometric range, if the throttle opening degree control means determines that a value of the torque is smaller than a first predetermined value, the throttle opening degree control means provides a first control of controlling the opening degree of the intake throttle valve such that the output shaft rotates at a constant first rotation number and, if the throttle opening degree control means determines that the value of the torque is larger than the first predetermined value, the throttle opening degree control means provides a second control of controlling the opening degree of the intake throttle valve such that the rotation number of the output shaft increases in accordance with a linear function as the value of the torque becomes larger than the first predetermined value.