GAS ENGINE

[Object] is to provide a gas engine capable of controlling an air-fuel ratio in response to a rapid change in the calorific value of a fuel gas. [Solution] is a gas engine 1 including an A/F valve 22, a solenoid valve 21, and a control unit 10 that performs perturbation with the solenoid valve 21, the control unit 10 being configured to calculate an average opening degree bn based on a predetermined number of maximal opening degrees bl and minimal opening degrees bs obtained during a perturbation control, perform a control of closing the A/F valve 22 when the average opening degree bn is lower than a target opening degree a that is preset, and perform a control of opening the A/F valve 22 when the average opening degree bn is equal to or higher than the target opening degree a, wherein the control unit 10 detects a current opening degree of the solenoid valve 21, and the control unit 10 determines that the current opening degree keeps being an upper limit opening degree bmax or a lower limit opening degree bmin of the solenoid valve 21 over a predetermined number of times, the control unit 10 replaces the average opening degree bn with the upper limit opening degree bmax or the lower limit opening degree bmin, for comparison against the target opening degree a.