A two-stroke internal combustion engine comprising: a piston disposed in a cylinder, reciprocating between a top dead center and a bottom dead center, and defining a combustion chamber; an exhaust port opening in the cylinder and opened and closed by the piston for discharging a burnt gas in the combustion chamber; a crank chamber receiving a fresh gas and pre-compressing the fresh gas by a descending movement of the piston; and a plurality of scavenging passages including scavenging ports for communicating with the combustion chamber and the crank chamber in the scavenging stroke and discharging the fresh gas pre-compressed in the crank chamber as a scavenging gas to the combustion chamber, the scavenging port opened and closed by the piston, wherein the scavenging passage included in the plurality of scavenging passages and connected to at least one of the scavenging ports constitutes a variable scavenging passage, wherein an upper end portion of the variable scavenging passage has a guide surface, the guide surface being constituted by a side wall surface of the variable scavenging passage on a cylinder intake side, and the guide surface defining a discharge direction of the scavenging gas discharged from a variable scavenging port connected thereto on a horizontal plane, wherein the guide surface includes at least a first guide portion defining a first discharge direction of the scavenging gas and a second guide portion defining a second discharge direction of the scavenging gas, and wherein the discharge direction of the scavenging gas is changed from the first discharge direction to the second discharge direction along the horizontal plane by the first and second guide portions in the scavenging stroke, the horizontal plane being orthogonal to a vertical movement of the piston. a piston disposed in a cylinder, reciprocating between a top dead center and a bottom dead center, and defining a combustion chamber; an exhaust port opening in the cylinder and opened and closed by the piston for discharging a burnt gas in the combustion chamber; a crank chamber receiving a fresh gas and pre-compressing the fresh gas by a descending movement of the piston; and a plurality of scavenging passages including scavenging ports for communicating with the combustion chamber and the crank chamber in the scavenging stroke and discharging the fresh gas pre-compressed in the crank chamber as a scavenging gas to the combustion chamber, the scavenging port opened and closed by the piston, wherein the scavenging passage included in the plurality of scavenging passages and connected to at least one of the scavenging ports constitutes a variable scavenging passage, wherein an upper end portion of the variable scavenging passage has a guide surface, the guide surface being constituted by a side wall surface of the variable scavenging passage on a cylinder intake side, and the guide surface defining a discharge direction of the scavenging gas discharged from a variable scavenging port connected thereto on a horizontal plane, wherein the guide surface includes at least a first guide portion defining a first discharge direction of the scavenging gas and a second guide portion defining a second discharge direction of the scavenging gas, and wherein the discharge direction of the scavenging gas is changed from the first discharge direction to the second discharge direction along the horizontal plane by the first and second guide portions in the scavenging stroke, the horizontal plane being orthogonal to a vertical movement of the piston.