A backflow prevention valve includes a cylindrical case member having a plurality of holes formed in an upper end at a periphery thereof, a valve member disposed in the case member to be movable in an axial direction thereof, an urging device disposed in the case member for urging the valve member upwardly, and a liquid inflow pipe extending outwardly from the case member. The valve member includes a sealing portion at an upper surface thereof for contacting a peripheral edge of an inflow opening to close the same, and a plurality of guiding arms provided on a peripheral edge of an upper surface of the valve member. The guiding arms are slidably disposed in the holes so that the valve member is supported and guided by the guiding arms passing through the holes.