To efficiently cool bearings or the like in a crankcase so as to prolong the lifespan by circulating cooling air by a cooling fan within the crankcase.

Ventilation openings 26, 27 for introducing ambient air as the cooling air are arranged in a front wall surface 2A and a rear wall surface 2B of the crankcase 2 to which the main bearings 4, 5 for supporting both ends of a crankshaft 3 are attached. Accordingly, the cooling air generated by the cooling fan 28 is introduced into the crankcase 2 and exhausted out of the crankcase 2 through the ventilation openings 26, 27, thereby cooling the main bearings 4, 5 or the like within the crankcase 2 by the cooling air. Further, by providing a cooling air guide 29, the cooling air can be supplied toward cylinders 6, 11, 16, 21 so that the cylinder tubes 7, 12, 17, 22 or the like increased in temperature by compression heat or frictional heat can be cooled.