An oil separator for separating oil in a blow-by gas for an internal combustion engine, comprising: a housing forming a flow passage of the blow-by gas, and defining an annular shoulder face facing a downstream side; a valve member disposed inside the flow passage, and movable between a blocking position abutting an end face thereof against the annular shoulder face to block the flow passage, and an open position separating the end face from the annular shoulder face to open the flow passage; a spring device urging the valve member toward the blocking position; and a reflux device provided inside the housing for refluxing the oil separated from the blow-by gas to a crank chamber, wherein the valve member includes a first side face facing a wall face of the flow passage at the downstream side of the annular shoulder face and spaced from the wall face at a predetermined interval, a spring constant of the spring device is set such that the valve member moves to the open position against an urging force of the spring device by a differential pressure inside the flow passage, and a relation wherein a gap between the end face of the valve member and the annular shoulder face in the open position is narrower than the interval between the first side face of the valve member and the wall face of the flow passage is always established regardless of a flow volume of the blow-by gas, characterized in that in the annular shoulder face, there is formed a projecting piece functioning as a valve seat, and formed in an annular shape coaxial with the annular shoulder face; and an inner peripheral face of the projecting piece is formed such that an inner diameter thereof increases as it goes downward, and in a vertical cross-sectional view, the inner peripheral face of the projecting piece has a convex shape obliquely downward on a center axis side of the cylinder; wherein the valve member comprises a circular plate portion having an approximately circular plate shape; and an upper face of the circular plate portion abuts against a lower end of the projecting piece; and the upper face of the circular plate portion has a slightly inclined umbrella shape, so that attached oil flows toward a circumferential edge to drop. a housing forming a flow passage of the blow-by gas, and defining an annular shoulder face facing a downstream side; a valve member disposed inside the flow passage, and movable between a blocking position abutting an end face thereof against the annular shoulder face to block the flow passage, and an open position separating the end face from the annular shoulder face to open the flow passage; a spring device urging the valve member toward the blocking position; and a reflux device provided inside the housing for refluxing the oil separated from the blow-by gas to a crank chamber, wherein the valve member includes a first side face facing a wall face of the flow passage at the downstream side of the annular shoulder face and spaced from the wall face at a predetermined interval, a spring constant of the spring device is set such that the valve member moves to the open position against an urging force of the spring device by a differential pressure inside the flow passage, and a relation wherein a gap between the end face of the valve member and the annular shoulder face in the open position is narrower than the interval between the first side face of the valve member and the wall face of the flow passage is always established regardless of a flow volume of the blow-by gas, characterized in that in the annular shoulder face, there is formed a projecting piece functioning as a valve seat, and formed in an annular shape coaxial with the annular shoulder face; and an inner peripheral face of the projecting piece is formed such that an inner diameter thereof increases as it goes downward, and in a vertical cross-sectional view, the inner peripheral face of the projecting piece has a convex shape obliquely downward on a center axis side of the cylinder; wherein the valve member comprises a circular plate portion having an approximately circular plate shape; and an upper face of the circular plate portion abuts against a lower end of the projecting piece; and the upper face of the circular plate portion has a slightly inclined umbrella shape, so that attached oil flows toward a circumferential edge to drop.