An engine comprising: a cylinder head, a cylinder head cover mounted above the cylinder head, a rocker arm covered by the cylinder head cover, and a breather chamber provided inside the cylinder head cover, wherein: as a front-rear direction is defined by a longitudinal direction of the cylinder head cover, the breather chamber has a blow-by gas inlet at one side in the front-rear direction, a blow-by gas outlet at another side in the front-rear direction, and an oil discharging guide chamber at an intermediate position in the front-rear direction, the blow-by gas inlet is opened on a bottom wall of the breather chamber, a peripheral wall of the oil discharging guide chamber is protruded downwardly from the bottom wall of the breather chamber between the rocker arm, which is provided at a side of the blow-by gas outlet, and the blow-by gas inlet; the breather chamber has an inlet side oil separating chamber having the blow-by gas inlet, and a blow-by gas bypass passage, the breather chamber is configured to guide blow-by gas coming out of the inlet side oil separating chamber to the oil discharging guide chamber via the blow-by gas bypass passage; and the breather chamber has a partition wall which separates the blow-by gas bypass passage and the oil discharging guide chamber, characterized in that: the breather chamber 4 has a separated oil guide passage having a start part provided in the inlet side oil separating chamber, an intermediate part provided at a start part of the blow-by gas bypass passage, and an end part provided below the partition wall such that the separated oil guide passage is led to the oil discharging guide chamber. a cylinder head, a cylinder head cover mounted above the cylinder head, a rocker arm covered by the cylinder head cover, and a breather chamber provided inside the cylinder head cover, wherein: as a front-rear direction is defined by a longitudinal direction of the cylinder head cover, the breather chamber has a blow-by gas inlet at one side in the front-rear direction, a blow-by gas outlet at another side in the front-rear direction, and an oil discharging guide chamber at an intermediate position in the front-rear direction, the blow-by gas inlet is opened on a bottom wall of the breather chamber, a peripheral wall of the oil discharging guide chamber is protruded downwardly from the bottom wall of the breather chamber between the rocker arm, which is provided at a side of the blow-by gas outlet, and the blow-by gas inlet; the breather chamber has an inlet side oil separating chamber having the blow-by gas inlet, and a blow-by gas bypass passage, the breather chamber is configured to guide blow-by gas coming out of the inlet side oil separating chamber to the oil discharging guide chamber via the blow-by gas bypass passage; and the breather chamber has a partition wall which separates the blow-by gas bypass passage and the oil discharging guide chamber, characterized in that: the breather chamber 4 has a separated oil guide passage having a start part provided in the inlet side oil separating chamber, an intermediate part provided at a start part of the blow-by gas bypass passage, and an end part provided below the partition wall such that the separated oil guide passage is led to the oil discharging guide chamber. as a front-rear direction is defined by a longitudinal direction of the cylinder head cover, the breather chamber has a blow-by gas inlet at one side in the front-rear direction, a blow-by gas outlet at another side in the front-rear direction, and an oil discharging guide chamber at an intermediate position in the front-rear direction, the blow-by gas inlet is opened on a bottom wall of the breather chamber, a peripheral wall of the oil discharging guide chamber is protruded downwardly from the bottom wall of the breather chamber between the rocker arm, which is provided at a side of the blow-by gas outlet, and the blow-by gas inlet; the breather chamber has an inlet side oil separating chamber having the blow-by gas inlet, and a blow-by gas bypass passage, the breather chamber is configured to guide blow-by gas coming out of the inlet side oil separating chamber to the oil discharging guide chamber via the blow-by gas bypass passage; and the breather chamber has a partition wall which separates the blow-by gas bypass passage and the oil discharging guide chamber, characterized in that: the breather chamber 4 has a separated oil guide passage having a start part provided in the inlet side oil separating chamber, an intermediate part provided at a start part of the blow-by gas bypass passage, and an end part provided below the partition wall such that the separated oil guide passage is led to the oil discharging guide chamber.