PURPOSE: To prevent choking of an air exhaust port and improve ventilation in a transmission case by forming an air exhaust port in the position, whose three sides are surrounded by a driving sheave, a driven sheave, and a V-velt wound around both of these sheaves in a horizontal view, situated below a transmission case on the external side of a V-velt.

CONSTITUTION: Inside a transmission case 3 extended fore and aft on the outside of a rear wheel 6 in a motor cycle a driving shaft on an engine side and a rear shaft 5 for the rear wheel 6 are arranged longitudinally while a driven shaft 7 is arranged between the driving shaft and the rear shaft. A driving sheave and a driven sheave 13 are supported by the driving shaft and the driven shaft 7 respectively, the rear shaft 5 is connected to the driven sheave 13 via the driven shaft 7, and a V-velt 14 is wound around the driving sheave and the driven sheave 13. In this constitution, an air exhaust port 33 is formed at the lower part of the transmission case 3 on more external side than the V-velt 14 being surrounded from three sides by the driving sheave, the driven sheave 13, and the V-velt 14 in a horizontal view. The air exhaust port 33 is opened downward from the inside of the transmission case 3.