To provide an air advanced introduction type two-cycle internal combustion engine capable of reducing a blowby amount as much as possible, improving fuel consumption and an output and reducing the generation of a harmful component, arranging the periphery of the engine rationally and compactly, and reducing the number of parts, weight, and a cost, and improving processing and assembly ability.

An air introduction passage 30 to introduce air (A) to scavenging passages 31, 31, 32, and 32 is formed in a mating surface (J) between a cylinder 10 and an upper crank case 11.