Provided is a valve device including a float valve (2), a case (3) accommodating the float valve (2) therein, and an outer member (4) accommodating the case (3) therein and having an attachment member (4g) to be attached to a fuel tank (T). The valve device constitutes part of a flow channel of the fuel tank (T). The valve device is configured such that part of a cylindrical seal (7) is sandwiched and held between an upper portion (3a) of the case (3), the portion (3a) having a first through hole (3b) formed therein, and an upper portion (4a) of the outer member (4), the portion (4a) having formed therein a second through hole (4b) that communicates with the outside of the tank (T), the cylindrical seal (7) being fit into the first through hole (3b) so that the inside of the seal communicates with the second through hole (4b). The valve device is also configured such that a lower end of the cylindrical seal (7) forms a valve seat (7e) of the float valve (2).