In a two-stroke-cycle gasoline engine, the scavenging flow plays a very important role in the combustion performance. The scavenging flow discharges the burned gas from the cylinder and draws the mixture of fresh air and gasoline into it. But the flow pattern in the loop-flow scavenging system of this type of engine is very complicated and measurement of the gas velocity and the gas concentration is difficult. Thus there is little information about the flow characteristics in the cylinder. The authors have developed a simulation method for the uniflow scavenging two-stroke-cycle diesel engine. In this report, we outline the development of this method, applying it to the loop-flow scavenging two-stroke cycle gasoline engine. The characteristics of the gas flow and the gas concentration in this engine are made clear by the simulation method.