PURPOSE: To provide a low and a high speed, and a timing of opening and closing intake and exhaust ports of a rotary valve used in a four- or six-cycle engine, which is suitable for the purpose and the speed of an engine, and which enhances the combustion efficiency.

CONSTITUTION: A gear for driving a rotary valve 5, and the drive part of the rotary part 5 are of a screw type, and the gear for driving the rotary valve 5 is moved right and left. Further, an intake and exhaust port 3 of the rotary valve 5 has a width which is larger than that of an intake and exhaust port 4 of a combustion chamber, and accordingly, the shape of the intake and exhaust port of the rotary valve 5 is changed so that the opening and closing timing is changed when the rotary valve 5 is moved right and left.