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This paper presents a new method of controlling the air-to-fuel ratio (A/F) in gas engines with three-way catalysts. In the method under discussion, the center of the λ-window is detected directly through the dynamic response of an O2 sensor installed at the rear of the catalysts. As the result, the A/F remains stable at the center of the λ-window for long periods of time, even in cases where static response characteristics of the O2 sensor change due to the deterioration of the O2 sensor, or in instances where the λ-window shifts or decreases due to the aging of the catalysts. The responses of the O2 sensor outputs have delay times in both the step changes of lean-to-rich and rich-to-lean A/F. Varying delay times were observed, and it was determined that the differences depended on the mean value of the A/F of the step change. Moreover, the delay times equalized when the mean A/F was at the center of the λ-window.