A two-stroke internal combustion engine comprising: a cylinder bore formed in a cylinder; a piston inserted into the cylinder bore and can move reciprocatably, the piston defining a cylinder chamber in the cylinder bore; a crankshaft configured to convert a reciprocating motion of the piston into a rotary motion; a crank chamber configured to house the crankshaft and receive a gas mixture fed from an intake system; a scavenging port that is open to the cylinder chamber and opened and closed by the piston; a scavenging passage, one end of which ranges to the scavenging port and the other end of which is open to the crank chamber; and an exhaust port configured to exhaust a combustion gas in the cylinder chamber to an outside and opened and closed by the piston, wherein the scavenging passage includes a wall surface that directs a flow of a main scavenging gas spouting from the scavenging port to the cylinder chamber, characterized in that the scavenging port is composed of a main port portion directly continuing to the scavenging passage and a port extension forming a port extended passage extending in a lateral direction from the main port portion, and the port extension extends from the main port portion to the opposite side of the exhaust port in a cylinder circumferential direction; a boundary portion, where the port extension and the scavenging passage are in contact with each other, is composed of an inclined surface that gradually approaches the cylinder chamber toward the intake system, the port extension has a relatively small passage cross section such that a flow of an incidental scavenging gas spouting to the cylinder chamber through the port extension is relatively high in speed compared with the main scavenging gas flow and the relatively high-speed incidental scavenging gas flow attracts the main scavenging gas flow, wherein the inclined surface of the boundary portion is composed of a curved surface having a convex shape toward the cylinder chamber. a cylinder bore formed in a cylinder; a piston inserted into the cylinder bore and can move reciprocatably, the piston defining a cylinder chamber in the cylinder bore; a crankshaft configured to convert a reciprocating motion of the piston into a rotary motion; a crank chamber configured to house the crankshaft and receive a gas mixture fed from an intake system; a scavenging port that is open to the cylinder chamber and opened and closed by the piston; a scavenging passage, one end of which ranges to the scavenging port and the other end of which is open to the crank chamber; and an exhaust port configured to exhaust a combustion gas in the cylinder chamber to an outside and opened and closed by the piston, wherein the scavenging passage includes a wall surface that directs a flow of a main scavenging gas spouting from the scavenging port to the cylinder chamber, characterized in that the scavenging port is composed of a main port portion directly continuing to the scavenging passage and a port extension forming a port extended passage extending in a lateral direction from the main port portion, and the port extension extends from the main port portion to the opposite side of the exhaust port in a cylinder circumferential direction; a boundary portion, where the port extension and the scavenging passage are in contact with each other, is composed of an inclined surface that gradually approaches the cylinder chamber toward the intake system, the port extension has a relatively small passage cross section such that a flow of an incidental scavenging gas spouting to the cylinder chamber through the port extension is relatively high in speed compared with the main scavenging gas flow and the relatively high-speed incidental scavenging gas flow attracts the main scavenging gas flow, wherein the inclined surface of the boundary portion is composed of a curved surface having a convex shape toward the cylinder chamber.