A cylinder block includes a scavenging passage, which extends between a combustion chamber and a crankcase chamber to communicate them together and has a scavenging port defined in the cylinder block so as to open at an inner peripheral surface of the cylinder block. An intake passage or exhaust passage, the scavenging passage including the scavenging port and a throughhole defined therebetween are formed by the use of a molding piece movable in a direction towards the fuel intake passage or exhaust passage and the throughhole is closed by a lid.