To simultaneously achieve prevention of malfunction of a valving element, and prevention of wear and operation sound of a sliding portion.

In a PCV valve (Positive Crankcase Ventilation) 40, a valving element 46 is reciprocatably accommodated in a case 42 having an inflow port 43 and an outflow port 44. A measuring portion 56 is formed by an inner wall of the case 42. A blow-by gas is measured by changing a position of a taper-shaped portion 62 of the valving element 46. The valving element 46 is provided with a plurality of downstream-side guide portions 70 radially projected from the taper-shaped portion, extended in the axial direction, and capable of being slidably kept into contact with the measuring portion 56 of the case 42. The valving element 46 is further provided with a plurality of upstream-side guide portions 65 radially projected from a basic end-side portion, and capable of being slidably kept into contact with an upstream-side passage wall surface 52 of the case 42. The downstream-side guide portions 70 and the upstream-side guide portions 65 are located on different positions in the circumferential direction of the valving element 46.