Among multiple scavenging passages 14 included in a cylinder, a scavenging passage connected to at least one scavenging port 16 constitutes a variable scavenging passage 14(ch). An upper end portion of the variable scavenging passage 14(ch) has a guide surface 50 defining a discharge direction of a scavenging gas discharged from a variable scavenging port 16(ch) connected thereto on a horizontal plane. The guide surface 50 includes at least a first guide portion 50(H) defining a first discharge direction of the scavenging gas and a second guide portion 50(L) defining a second discharge direction of the scavenging gas. The discharge direction of the scavenging gas is changed from the first discharge direction to the second discharge direction on the horizontal plane by the first and second guide portions 50(H) and 50(L) in the scavenging stroke.