To provide a four cycle engine, allowing the use of a three-way catalyst with combustion near a stoichiometric air-fuel ratio by greatly reducing pumping loss in an intake stroke.

An intake leak opening portion 2 is provided in a combustion chamber 1, where an intake leak valve 3 is installed. An intake leak pipe 4 is connected to the intake leak opening portion 2. A throttle valve 5 for controlling an output in linkage with acceleration is provided in the intake leak pipe 4. The intake leak valve 3 is set to be opened/closed at a reasonable timing between the late stage of an intake stroke and the completion of a compression stroke. The throttle valve 5 in the intake leak pipe 4 is used for controlling the amount of intake air to be leaked from the combustion chamber 1, namely, controlling the amount of intake air to reside in the combustion chamber 1, with the result that the operating output of the engine is controlled.