An oil separator for removing oil from blow-by gas of an internal combustion engine, comprising: a blow-by gas passage including an upstream passage and a downstream passage connected to a downstream end of the upstream passage and provided with a separation wall for changing a flow direction of the blow-by gas that has passed through the upstream passage; wherein the separation wall includes a first surface (40, 78) opposing the upstream passage, and a second surface adjoining the first surface (40, 78) on a downstream side thereof, wherein the upstream passage includes a pair of side walls having mutually opposing wall surfaces, a rear wall extending to the separation wall and a front wall having a free end opposing the separation wall at a gap, characterised in that: the second surface defines a planar surface extending substantially perpendicularly to the upstream passage, in that the first surface (40, 78) defines a planar surface or a concave curved surface more proximate to the upstream passage than a planar extension of the second surface, in that the oil separator further comprises a narrowed passage portion which is narrowed relative to parts that are immediately upstream and downstream of the narrowed portion, the narrowed portion being defined by the side walls, a free end side of the rear wall, and the free end side of the front wall, in that a boundary line between the first surface (40, 78) and the second surface is located more downstream than an intersection line between the separation wall and an extension plane of a rear surface of the front wall, and in that, in use, at least a part of oil particles moving along the first surface (40, 78) collides with, and is trapped by, the second surface. a blow-by gas passage including an upstream passage and a downstream passage connected to a downstream end of the upstream passage and provided with a separation wall for changing a flow direction of the blow-by gas that has passed through the upstream passage; wherein the separation wall includes a first surface (40, 78) opposing the upstream passage, and a second surface adjoining the first surface (40, 78) on a downstream side thereof, wherein the upstream passage includes a pair of side walls having mutually opposing wall surfaces, a rear wall extending to the separation wall and a front wall having a free end opposing the separation wall at a gap, characterised in that: the second surface defines a planar surface extending substantially perpendicularly to the upstream passage, in that the first surface (40, 78) defines a planar surface or a concave curved surface more proximate to the upstream passage than a planar extension of the second surface, in that the oil separator further comprises a narrowed passage portion which is narrowed relative to parts that are immediately upstream and downstream of the narrowed portion, the narrowed portion being defined by the side walls, a free end side of the rear wall, and the free end side of the front wall, in that a boundary line between the first surface (40, 78) and the second surface is located more downstream than an intersection line between the separation wall and an extension plane of a rear surface of the front wall, and in that, in use, at least a part of oil particles moving along the first surface (40, 78) collides with, and is trapped by, the second surface. the second surface defines a planar surface extending substantially perpendicularly to the upstream passage, in that the first surface (40, 78) defines a planar surface or a concave curved surface more proximate to the upstream passage than a planar extension of the second surface, in that the oil separator further comprises a narrowed passage portion which is narrowed relative to parts that are immediately upstream and downstream of the narrowed portion, the narrowed portion being defined by the side walls, a free end side of the rear wall, and the free end side of the front wall, in that a boundary line between the first surface (40, 78) and the second surface is located more downstream than an intersection line between the separation wall and an extension plane of a rear surface of the front wall, and in that, in use, at least a part of oil particles moving along the first surface (40, 78) collides with, and is trapped by, the second surface.