To suppress erosion and elution of a humidifier housing that are caused by humidification fluid flowing through a flow path in a humidifier consisting of conductive materials.

A humidifier 100 is interposed among many fuel cells. The humidifier 100 includes a hollow string membrane 106 for circulating the fluid being humidified to be supplied to the fuel cells toward the inside while transmitting vapor, and conductive housings 110 and 150 having a flow path 100D for circulating the humidification fluid with vapor partial pressure higher than that of the fluid being humidified toward the outside of the hollow string membrane 106. An erosion protection layer is formed on the inner wall side of the flow path 100D.