A gas engine comprising: a first valve and a second valve provided in parallel on a supply path of fuel gas, the first valve having a responsiveness being lower and a fuel flow rate adjustment range being wider than the second valve, and the second valve having a responsiveness being higher and a fuel flow rate adjustment range being narrower than the first valve; and a control unit configured to perform perturbation using the second valve at the time of stoichiometric operation by moving the second valve from a predetermined opening degree to a lean side where the opening degree becomes smaller and a rich side where the opening degree becomes larger alternately at a predetermined pitch while the first valve is being opened at a predetermined opening degree, wherein, during an actual operation in a period in which an operation condition of the gas engine is deemed to be stable under the perturbation control by the second valve at a predetermined engine rotational speed and with a predetermined load, when an average value of outputs obtained from an oxygen sensor (31,32) disposed on an exhaust passage of the gas engine deviates from an output target value a of the oxygen sensor (31,32), the output target value a being set in the control unit under the operation condition, the control unit is configured to adjust an opening degree of the first valve so that the output average value b equals the output target value a, and wherein the control unit calculates the output average value b by extracting a maximum output and a minimum output of the oxygen sensor (31,32) respectively corresponding to a maximum opening degree and a minimum opening degree in the perturbation control by the second valve during the actual operation. a first valve and a second valve provided in parallel on a supply path of fuel gas, the first valve having a responsiveness being lower and a fuel flow rate adjustment range being wider than the second valve, and the second valve having a responsiveness being higher and a fuel flow rate adjustment range being narrower than the first valve; and a control unit configured to perform perturbation using the second valve at the time of stoichiometric operation by moving the second valve from a predetermined opening degree to a lean side where the opening degree becomes smaller and a rich side where the opening degree becomes larger alternately at a predetermined pitch while the first valve is being opened at a predetermined opening degree, wherein, during an actual operation in a period in which an operation condition of the gas engine is deemed to be stable under the perturbation control by the second valve at a predetermined engine rotational speed and with a predetermined load, when an average value of outputs obtained from an oxygen sensor (31,32) disposed on an exhaust passage of the gas engine deviates from an output target value a of the oxygen sensor (31,32), the output target value a being set in the control unit under the operation condition, the control unit is configured to adjust an opening degree of the first valve so that the output average value b equals the output target value a, and wherein the control unit calculates the output average value b by extracting a maximum output and a minimum output of the oxygen sensor (31,32) respectively corresponding to a maximum opening degree and a minimum opening degree in the perturbation control by the second valve during the actual operation.