An air leading-type stratified scavenging two-stroke internal-combustion engine comprising: an air port ( 4a, 38a) that opens in a cylinder wall ( 2,28) and is opened/closed by a piston; a scavenging channel ( 6, 30, 32) including a scavenging port ( 6a, 30a, 32a) that opens in the cylinder wall ( 2, 28) and is opened/closed by the piston, the scavenging channel ( 6, 30, 32) communicating with a crankcase; a piston groove ( 8, 22) formed in a peripheral surface of the piston, the piston groove ( 8, 22) enabling the air port ( 4a, 38a) and the scavenging port ( 6a, 30a, 32a) to communicate with each other; and a gas venting port ( 10, 46) that communicates with the scavenging channel (6, 30, 32), opens in the cylinder wall ( 2, 28) independently from the scavenging port (6a, 30a, 32a) and is opened/closed by the piston, wherein the gas venting port ( 10, 46) is positioned on the crankcase side that is lower than the scavenging port ( 6a, 30a, 32a) in a cylinder axis direction, wherein in a course of the piston moving up toward the top dead center, before the piston groove ( 8, 22) that is in communication with the air port ( 4a, 38a) comes into communication with the scavenging port ( 6a, 30a, 32a), the piston groove ( 8, 22) that is in communication with the air port ( 4a, 38a) comes into communication with the gas venting port ( 10, 46), wherein in a course of the piston moving from the bottom dead center to the top dead center, the piston groove ( 8, 22) is brought into communication with the gas venting port ( 10, 46) and then is brought into communication with the scavenging port ( 6a, 30a, 32a), and characterised in that, the gas venting port (10,46) communicates with the crankcase via the scavenging channel (6, 30,32), and wherein, when the piston is at the top dead center, the piston groove ( 8, 22) is not in communication with the air port ( 4a, 38a). an air port ( 4a, 38a) that opens in a cylinder wall ( 2,28) and is opened/closed by a piston; a scavenging channel ( 6, 30, 32) including a scavenging port ( 6a, 30a, 32a) that opens in the cylinder wall ( 2, 28) and is opened/closed by the piston, the scavenging channel ( 6, 30, 32) communicating with a crankcase; a piston groove ( 8, 22) formed in a peripheral surface of the piston, the piston groove ( 8, 22) enabling the air port ( 4a, 38a) and the scavenging port ( 6a, 30a, 32a) to communicate with each other; and a gas venting port ( 10, 46) that communicates with the scavenging channel (6, 30, 32), opens in the cylinder wall ( 2, 28) independently from the scavenging port (6a, 30a, 32a) and is opened/closed by the piston, wherein the gas venting port ( 10, 46) is positioned on the crankcase side that is lower than the scavenging port ( 6a, 30a, 32a) in a cylinder axis direction, wherein in a course of the piston moving up toward the top dead center, before the piston groove ( 8, 22) that is in communication with the air port ( 4a, 38a) comes into communication with the scavenging port ( 6a, 30a, 32a), the piston groove ( 8, 22) that is in communication with the air port ( 4a, 38a) comes into communication with the gas venting port ( 10, 46), wherein in a course of the piston moving from the bottom dead center to the top dead center, the piston groove ( 8, 22) is brought into communication with the gas venting port ( 10, 46) and then is brought into communication with the scavenging port ( 6a, 30a, 32a), and characterised in that, the gas venting port (10,46) communicates with the crankcase via the scavenging channel (6, 30,32), and wherein, when the piston is at the top dead center, the piston groove ( 8, 22) is not in communication with the air port ( 4a, 38a).