A humidifying device (10), for humidifying a gas with the water vapor contained in air, comprises: a hollow fiber bundle (14) formed by bundling a plurality of hollow fibers (14a) permeable by water vapor, the hollow fibers being orientated in a direction of a predetermined axis a housing (12) having a space for accommodating the hollow fiber bundle (14), and having an introduction port (12a) for the gas to be humidified, communicating to bores of the hollow fibers, a discharging port (12b) for the gas to be humidified, communicating to the interior spaces of the hollow fibers, an air inlet (12c) communicating to a space in the housing external of the hollow fibers, and an air exit (12d) communicating to the space in the housing external of the hollow fibers and blowing means (16) arranged at the air inlet (12c) of the housing for introducing air into the housing, wherein a ratio between a sum of the cross-sectional areas of the hollow fibers taken along a plane perpendicular to the axis, and the cross-sectional area of an air passage, is set within a range from 0.1 to 0.7, the cross-sectional area of the air passage being obtained by subtracting the sum of the cross-sectional areas of the hollow fibers from the cross-sectional area of the space of the housing (12) taken along a plane perpendicular to the axis.
