This paper deals with the separation of short-circuit gas from exhaust gas in a two-stroke cycle gasoline engine and the measures to effectively utilize the short-circuit gas. For trial manufacture of the engine, the slide shutter is kept on an open port, and is set on the upper plane of the piston. This is done in order to get control of the exhaust port. The piston moves up and down the side of the exhaust port, which is divided into 3 section areas: upper, middle and lower. This mechanism in the lower level was able to separate the fresh and burned gas concentrations, at a high level, from the exhaust gas. We succeeded in achieving a firing-run on a separated and set four-stroke cycle gasoline engine by taking advantage of this gas. The data include, among others, the relationship between the trial manufacture of the engine and the general purpose engine in performance, specific fuel consumption and emission gases.