PURPOSE: To increase a blow-by angle for scavenging, so as to improve filling efficiency of scavenging, and to reduce a residual exhaust by arranging an expanded part that has a top in a specified interval from a wall surface of a cylinder, on a specified portion of a scavenging hole opened to the right and to the left of the cylinder.

CONSTITUTION: In a cylinder 1, an exhaust hole 2 is opened in front, and a scavenging hole 3 on each side surface. The opening ports 2, 3 are opened and closed according to the reciprocation of a piston 5 arranged in the cylinder 1. In the structure, on an upper end part in the vicinity of the opening port of the cylinder 1 in the scavenging hole 3, an expanded part 6 is arranged. A top 7 of the expanded part 6 is set in an interval T of a ratio of 1/10-1/5 to an inner diameter of the cylinder 1. A blow-up angle of scavenging is thus increased by +β. Filling efficiency of scavenging is increased thereby, and by decreasing a residual exhaust, horsepower can be improved.