In order to develop a clean engine, the sub-chamber type gasoline engine was suggested. The engine, however, has a serious fault such as the remaining of residual gas in the sub-chamber. In this experimental study, the direct CNG injection into the sub-chamber attempts to scavenge the residual gas. The CVC (constant volume chamber) is divided into sub-chamber and main-chamber, and used as the main experimental apparatus. Because each combustion chamber of the CVC has an injector, the injector can be used freely at the same time or individually. Therefore, the scavenging ratio can be altered by a change in the injection times into the sub-chamber. By the experimental results, when injecting all the fuel into the sub-chamber, the combustion duration is shorten by 30%, compared with the injection into the main-chamber. Also, though the residual gas exist in the CVC, when the injection times increase more and more into the sub-chamber, good combustion characteristics are obtained.