A breather chamber is located in a blow-by gas passage to separate oil from blow-by gas. A partitioning member partitions the interior of the breather chamber into a plurality of spaces. The partitioning member is arranged such that the spaces extend along the flowing direction of the blow-by gas and that an adjacent pair of the spaces are connected at ends in the flowing direction. A separating member is provided in at least one of the spaces. The separating member obstructs the flow of the blow-by gas in the associated space, thereby separating oil from the blow-by gas. Therefore, oil is separated from the blow-by gas securely and is prevented from being brought out of the breather chamber.