Breather structure of motorcycle engine

[Object] To prevent an oil from flowing into a breather outlet (102) in a motorcycle engine (E) even when a vehicle body is considerably leaned with the engine in operation, the motorcycle engine (E) including: an engine body (20) supported by a vehicle-body frame (F) such that an axis (C) of a cylinder bore (26) is arranged in parallel to a vertical plane (VP) in a vehicle-longitudinal direction; a power-transmission-member chamber (61) for moving an endless power transmission member (60) of a timing transmission device (57) provided between a crankshaft (25) and a camshaft (52) constituting a part of a valve system (51), the power-transmission-member chamber (61)formed in the engine body (20) at one side in a lateral direction of a vertical plane (VP) including the axis (C) of the cylinder bore (26); and a breather outlet (102) provided in a cylinder head (23) or a head cover (24), the breather outlet (102) configured to exhaust a blow-by gas introduced into a valve chamber (50) from a crank chamber (27) to the outside of the engine body (20). [Solving Means] The breather passage (85) connecting the valve chamber (50) and the crank chamber (27) is formed in the engine body in such a manner as to be arranged on an opposite side of the vertical plane VP including the axis (C) of the cylinder bore (26) to the power-transmission-member chamber (61).