PURPOSE: To improve engine efficiency, reduce the size, and improve low load operation by arranging a large diameter and single exhaust valve at a center of the cylinder head, and arranging a plurality of scavenging valves concentrically therearound.

CONSTITUTION: In a cylinder 1 of an engine, a large diameter and single exhaust valve 2 is arranged on a center of a cylinder head, while a plurality of scavenging valves 3 are concentrically arranged therearound. An intake manifold of the scavenging valve 3 is operated for flowing scavenge air in a tangential direction of the cylinder 1, and causing swirl scavenging. A guide vane 4 is a blade for flowing the scavenge air in the tangential direction of the cylinder 1. The scavenge air returns along an inner wall through interlocking with the intake manifold. Scavenging is performed when a piston 5 reaches a bottom dead center. The scavenge air turns and descends around the cylinder 1, strikes against the piston 5 followed by inversion, turns and ascends at a center portion.