A valve device , comprising: a housing in which, via a partition wall , a valve chamber (V) configured to communicate with an inside of a fuel tank is provided and a vent chamber (R) configured to communicate with an outside of the fuel tank is provided, and a vent hole through which the valve chamber (V) and the vent chamber (R) communicate with each other is provided in the partition wall ; and a float valve that is accommodated in the valve chamber (V) so as to be able to move up and down and that opens and closes the vent hole , wherein a valve seat 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 to close and open the first opening and the second openings , is disposed on the float valve , 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 , 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 (V) configured to communicate with an inside of a fuel tank is provided and a vent chamber (R) configured to communicate with an outside of the fuel tank is provided, and a vent hole through which the valve chamber (V) and the vent chamber (R) communicate with each other is provided in the partition wall ; and a float valve that is accommodated in the valve chamber (V) so as to be able to move up and down and that opens and closes the vent hole , wherein a valve seat 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 to close and open the first opening and the second openings , is disposed on the float valve , 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 , 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 . <EOS>