A stratified scavenging two-stroke internal combustion engine comprising: a piston valve-type engine body; a carburetor being interposed between an engine body and an air cleaner; a throttle valve (6, 204) that is arranged in the carburetor and is composed of a plate butterfly valve; a main nozzle (24, 202) that discharges fuel toward a plate surface of the throttle valve (6, 204) in a fully-open state; and an intake member that connects the carburetor and the engine body, wherein the intake member includes a fresh air passage and an air-fuel mixture passage and is capable of communicating with a crankcase of the engine body through an air-fuel mixture port, the engine body comprising: a scavenging passage that feeds an air-fuel mixture in the crankcase to a combustion chamber to perform scavenging; an air port that receives fresh air from the fresh air passage; and a piston groove that is provided in a peripheral surface of a piston so as to fill an upperportion of the scavenging passage with the fresh air from the air port, wherein an upstream side of the throttle valve is composed of an open space with no dividing wall, wherein the crankcase starts to be filled with the air-fuel mixture from the air-fuel mixture passage before a timing at which the upper portion of the scavenging passage starts to be filled with the fresh air with the piston groove and the scavenging passage communicating with each other during an upstroke of the piston; and characterized in that a venturi portion, which is a portion constricting a passage in the carburetor and arranged upstream of the throttle valve (6, 204) to utilize the Bernoulli's principle, is omitted from the carburetor. a piston valve-type engine body; a carburetor being interposed between an engine body and an air cleaner; a throttle valve (6, 204) that is arranged in the carburetor and is composed of a plate butterfly valve; a main nozzle (24, 202) that discharges fuel toward a plate surface of the throttle valve (6, 204) in a fully-open state; and an intake member that connects the carburetor and the engine body, wherein the intake member includes a fresh air passage and an air-fuel mixture passage and is capable of communicating with a crankcase of the engine body through an air-fuel mixture port, the engine body comprising: a scavenging passage that feeds an air-fuel mixture in the crankcase to a combustion chamber to perform scavenging; an air port that receives fresh air from the fresh air passage; and a piston groove that is provided in a peripheral surface of a piston so as to fill an upperportion of the scavenging passage with the fresh air from the air port, wherein an upstream side of the throttle valve is composed of an open space with no dividing wall, wherein the crankcase starts to be filled with the air-fuel mixture from the air-fuel mixture passage before a timing at which the upper portion of the scavenging passage starts to be filled with the fresh air with the piston groove and the scavenging passage communicating with each other during an upstroke of the piston; and characterized in that a venturi portion, which is a portion constricting a passage in the carburetor and arranged upstream of the throttle valve (6, 204) to utilize the Bernoulli's principle, is omitted from the carburetor. a scavenging passage that feeds an air-fuel mixture in the crankcase to a combustion chamber to perform scavenging; an air port that receives fresh air from the fresh air passage; and a piston groove that is provided in a peripheral surface of a piston so as to fill an upperportion of the scavenging passage with the fresh air from the air port, wherein an upstream side of the throttle valve is composed of an open space with no dividing wall, wherein the crankcase starts to be filled with the air-fuel mixture from the air-fuel mixture passage before a timing at which the upper portion of the scavenging passage starts to be filled with the fresh air with the piston groove and the scavenging passage communicating with each other during an upstroke of the piston; and characterized in that a venturi portion, which is a portion constricting a passage in the carburetor and arranged upstream of the throttle valve (6, 204) to utilize the Bernoulli's principle, is omitted from the carburetor.