The characteristics of set-on and set-off operations by input light of the same wavelength have been observed in a side light-injection MQW bistable laser. In this structure, the main bistable laser was located perpendicular to two wave-guides for amplifying input light. Two intersections, the gain quenching region and the saturable absorption region, were spatially separated. Input light of 40μW results in saturable absorption in one intersection based at +0.65V, and 570μW causes gain quenching in the other intersection biased at +0.93 V.