This specification discloses a humidifier that attains the inhibition of the damaging of a steam permeable membrane at low temperatures, and a fuel cell system. The humidifier has a steam permeable membrane therein. In the humidifier, an oxidizing gas and an oxidizing off gas with a humidity higher than that of the oxidizing gas are introduced, and the oxidizing gas is humidified by the oxidizing off gas via the steam permeable membrane. The steam permeable membrane is located at a position lower than an inlet for the oxidizing gas but higher than an outlet therefor. Further, the steam permeable membrane is located at a position lower than an inlet for the oxidizing off gas but higher than an outlet therefor.