A valve device forming a portion of a ventilation flow channel of a fuel tank (T) includes a float valve (2), a case (3) housing the float valve (2), and an outside member (4) housing the case (3) and including an attachment portion (4g) to a fuel tank (T) side. One portion of a cylindrical seal member (7) fitted in a first through hole (3b) to communicate an inner portion to a second through hole (4b) is pinched between a top portion (3a) of the case (3) wherein the first through hole (3b) is formed, and a top portion (4a) of the outside member (4) wherein the second through hole (4b) communicating to an outside of the tank (T) is formed, and a lower end of the cylindrical seal member (7) becomes a valve seat (7e) of the float valve (2).