A blow-by gas introducing passage having a blow-by gas inlet port, an air-cleaner-side breather passage that extends upwardly as branched from the blow-by gas introducing passage so as to communicate with an intake system, and a case-side breather passage that extends downwardly as branched from the blow-by gas introducing passage so as to communicate with an oil chamber are provided to an inner surface of a side cover that is detachably mounted to a timing-gear chamber communicating with a crank chamber. The blow-by gas introducing passage, the air-cleaner-side breather passage, and the case-side breather passage are formed integral with the side cover.