While providing 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 [Nm], 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. If the throttle opening degree control means determines that the value of the torque is larger than A [Nm], 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 becomes larger.