PURPOSE: To provide a unit swing engine of a motorcycle having a breather device which is of simple structure and which is easily attachable to and detachable from a cylinder head.

CONSTITUTION: A unit swing engine 8 of a motorcycle is furnished with a breather device integrating an engine 12 and a transmission case 14, supporting them on a car body free to vertically oscillate, circulating engine oil from a crank room 45 in a cam chamber 40 formed on a cylinder head 19 of the engine 12 and communicating the cam chamber 40 through to the air on the cylinder head 19. This breather device is constituted of a cover body installed on the cylinder head 19 free to connect and disconnect, a regulating member to regulate provided in the inside of this cover body and to regulate passing of engine oil and a communicating passage to communicate a space partitioned on the cover body by this regulating member.