Cooling structure for belt type stepless change

This invention relates to a cooling structure of a continuously variable belt transmission, being capable of preventing invasion of water or ash, advanced with a small quantity of components, small and simple structure, and beautiful appearance. In a power unit (20) installed in an automatic bicycle, a continuously variable belt transmission (46) is covered, while a transmission case cover (36) covers the transmission case (31L) from a side so as to form a speed change chamber (46C); a cooling fan (48F) is disposed on a shaft end of a crank shaft (40); a cooling air guide-in hole (36i) is disposed on a part of the transmission case cover (36) in correspondence with the cooling fan (48F), so as to guide the cooling air to the speed change chamber (46C), where the cooling air guide-in hole (36i) of the transmission case cover (36) and periphery thereof are covered from a side by a side cover part (92); an outer air guide-in hole (95) with an opening upwards is disposed on the end face of circumferential wall of