Abstract: A small and simple humidifier includes at least one hollow fiber membrane module formed by inserting into a cylindrical housing, both ends of which are opened, a bunch of water-permeable hollow fiber membranes along the axis of the housing, fixing both ends of the bunch at the outer circumference sides onto both ends of the housing at the inner circumference sides using a resin to thereby block the ends of the housing so as to form a plurality of peripheral apertures which are communicated within the housing at several intervals in the circumferential direction. The humidifier also includes a first passage which allows gas for flowing from one end of the peripheral apertures to the other end thereof serving as an inlet and an outlet, and a second passage which allows gas for flowing from one end to the other end of the bunch of the hollow fiber membrane. The first passage and the second passage are sectioned by a plurality of overlapped plates in an indentation state.