A humidifier includes a second housing. A water storage tank is in a housing space of the second housing. The tank is detachable through an attachment/detachment port of the second housing. A third introduction hole communicates an inside and an outside of the second housing with each other in the second housing. A first tank hole communicates an inside and an outside of the tank with each other in the tank. Between the second housing and the tank, a first seal member separates a first downstream side flow path leading to an inside of the tank through the third introduction hole and the first tank hole, from an external space of the second housing. The first seal member extends along an inner surface of the second housing to entirely surround the tank, when viewed from a top of the tank.