PURPOSE: To allow a four cycle engine favourably corresponding to the mirror cycle by shaping the inner shape (sectional shape) of a rotary valve to an H- shape and forming an air port which is opened within a specific angular range even after the transition to the compression stroke.

CONSTITUTION: In order to allow a four-cycle engine to correspond to the mirror cycle, a rotary valve 2 is used in substitution for a piston valve. In this case, the inner shape (sectional shape) of the rotary valve 2 is formed into an H shape. An air port 22 which is opened within an angular range of 30-90° even after the transition to the compression stroke is arranged. Further, at the air port 22, a vacant space is formed. In the case of multicylinder, each vacant space is connected to one space. When the vacant space is connected to other cylinders, the stroke of a piston 6 which differs in each cylinder of four cylinders or more, i.e., each stroke of explosion 17, intake 18, compression 19, and exhaust 20 is executed. Accordingly, the prevention for the counterflow of mixture gas or the prevention of waste consumption of fuel is achieved, and the correspondence with the mirror cycle is enabled.