Gas engine

A gas engine is provided to control an air-fuel ratio in response to changes in composition of fuel gas. A gas engine 1 includes: an A/F valve 22; a solenoid valve 21; and a control unit 10 configured to perform perturbation using the solenoid valve 21 and set a relationship between an air-fuel ratio and respective opening degrees of the solenoid valve 21 and the A/F valve 22 under a specific engine operation condition using standard fuel gas. During an actual operation in a period in which the engine operation condition is deemed to be stable, when an opening-degree average value b of the solenoid valve 21 deviates from an opening-degree target value a of the solenoid valve 21 set in the control unit 10 under the above operation condition, the control unit 10 adjusts the opening degree of the A/F valve 22 so that the opening-degree average value b equals the opening-degree target value a.