To increase the wind volume of the cooling wind flowing through a radiator and to enhance the cooling performance of the radiator by providing a wind exhaust duct using a crankcase of an internal combustion engine. A cooling device of the internal combustion engine includes a radiator arranged lateral to crankcases. A wind exhaust duct exhausts the cooling wind passed through the radiator to the atmosphere. The wind exhaust duct is extends along a rotational axis of the crankshaft at the upper portion of the crankcases and the upper portion of the transmission case.