A valve device (10, 10A), comprising: a housing in which, via a partition wall, a valve chamber configured to communicate with an inside of a fuel tank is provided and a vent chamber configured to communicate with an outside of the fuel tank is provided, and a vent hole through which the valve chamber and the vent chamber communicate with each other is provided in the partition wall; and a float valve (60, 60A) that is accommodated in the valve chamber so as to be able to move up and down and that opens and closes the vent hole, wherein a valve seat (31, 90) is formed on a valve chamber side fo of the partition wall, and an opening communicating with the vent hole is provided on an inner side of the valve seat, wherein the opening includes a first opening and a plurality of second openings extending outward in a slit shape from an outer periphery of the first opening, and wherein a seal part having elasticity that contacts with and separates from the valve seat (31, 90) to close and open the first opening and the second openings, is disposed on the float valve (60, 60A), wherein a wall protrudes downward from the valve chamber side of the partition wall, and a tip end portion of the wall in a protruding direction constitutes the valve seat (31, 90), characterized in that the wall is a cylindrical wall, the cylindrical wall includes a first wall portion and a plurality of second wall portions extending outward from an outer periphery of the first wall portion, and the first opening is provided on an inner side of the first wall portion, and the plurality of second openings is provided on an inner side of each second wall portion. a housing in which, via a partition wall, a valve chamber configured to communicate with an inside of a fuel tank is provided and a vent chamber configured to communicate with an outside of the fuel tank is provided, and a vent hole through which the valve chamber and the vent chamber communicate with each other is provided in the partition wall; and a float valve (60, 60A) that is accommodated in the valve chamber so as to be able to move up and down and that opens and closes the vent hole, wherein a valve seat (31, 90) is formed on a valve chamber side fo of the partition wall, and an opening communicating with the vent hole is provided on an inner side of the valve seat, wherein the opening includes a first opening and a plurality of second openings extending outward in a slit shape from an outer periphery of the first opening, and wherein a seal part having elasticity that contacts with and separates from the valve seat (31, 90) to close and open the first opening and the second openings, is disposed on the float valve (60, 60A), wherein a wall protrudes downward from the valve chamber side of the partition wall, and a tip end portion of the wall in a protruding direction constitutes the valve seat (31, 90), characterized in that the wall is a cylindrical wall, the cylindrical wall includes a first wall portion and a plurality of second wall portions extending outward from an outer periphery of the first wall portion, and the first opening is provided on an inner side of the first wall portion, and the plurality of second openings is provided on an inner side of each second wall portion.