A method for humidifying a supply gas of a fuel cell, comprising transferring 71% or more of the water content from a low-pressure first gas at 1 to 4 atm to a low-pressure second gas at 1 to 4 atm with the use of a humidifying apparatus fabricated by loading a hollow fiber membrane element in which a hollow fiber membrane bundle comprising a large number of hollow fiber membranes of a polyimide resin is anchored with tubesheets at both ends with the hollow fiber membranes being in the opened state, into a container having at least a supply port of a first gas, a discharge port of the first gas, a supply port of a second gas and a discharge port of the second gas, such that the space communicating with the hollow side of the hollow fiber membranes is isolated from the space communicating with the outer side of the hollow fiber membranes, wherein (a) the inner diameter of the hollow fiber membranes is from more than 400 μm to less than 1,500 μm, (b) the water vapour permeation rate (P' H2O) of the hollow fiber membranes is 1.67 x 10 -7 mol/(m 2 Pa·s) (0.5 × 10 -3 cm 3 /cm 2 · spec · cm Hg) or more, when measured with nitrogen gas having a water vapor concentration of about 23 vol% at a temperature of 80°C, (c) the ratio (P' H2O/ P' O2) of the water vapour permeation rate to the oxygen gas permeation rate of the hollow fiber membranes is 10 or more, when measured with nitrogen gas having a water vapor concentration of about 23 vol% and with pure oxygen gas, respectively, at a temperature of 80°C, (d) the elongation at tensile break of the hollow fiber membranes after hot water treatment in hot water at 100°C for 50 hours and subsequent drying at 100°C is 80% or more of that before the hot water treatment, when measured at a temperature of 23°C and an elongation rate of 10 mm/min, (e) assuming that the effective length of the hollow fiber membrane element is L and the inner diameter of the container into which said hollow fiber membrane element is loaded is D, L/D is from 2 to 3.2, and (f) the membrane filling percentage of the hollow fiber membrane bundle constituting the hollow fiber membrane element is from 35 to 55%. (a) the inner diameter of the hollow fiber membranes is from more than 400 μm to less than 1,500 μm, (b) the water vapour permeation rate (P' H2O) of the hollow fiber membranes is 1.67 x 10 -7 mol/(m 2 Pa·s) (0.5 × 10 -3 cm 3 /cm 2 · spec · cm Hg) or more, when measured with nitrogen gas having a water vapor concentration of about 23 vol% at a temperature of 80°C, (c) the ratio (P' H2O/ P' O2) of the water vapour permeation rate to the oxygen gas permeation rate of the hollow fiber membranes is 10 or more, when measured with nitrogen gas having a water vapor concentration of about 23 vol% and with pure oxygen gas, respectively, at a temperature of 80°C, (d) the elongation at tensile break of the hollow fiber membranes after hot water treatment in hot water at 100°C for 50 hours and subsequent drying at 100°C is 80% or more of that before the hot water treatment, when measured at a temperature of 23°C and an elongation rate of 10 mm/min, (e) assuming that the effective length of the hollow fiber membrane element is L and the inner diameter of the container into which said hollow fiber membrane element is loaded is D, L/D is from 2 to 3.2, and (f) the membrane filling percentage of the hollow fiber membrane bundle constituting the hollow fiber membrane element is from 35 to 55%.