NOVELTY - The humidification system comprises membrane humidifier (21) having hollow fiber membrane (21a) in housing (I), another membrane humidifier (22) having hollow fiber membrane (22a) in housing (II), and flow path switching valve to selectively open and close discharge flow path of humidified air to be supplied to fuel cell stack. The dry air passes through fiber membranes and supersaturated humid air discharged from fuel cell stack of fuel cell passes through housings (I,II) to carry out moisture exchange between dry air and humid air and produce humidified air.    USE - Humidification system is used for fuel cell used in vehicles such as buses, trucks, boats, ships and aircraft for generating electric power.    ADVANTAGE - The humidification system adjusts humidity of dry air supplied to fuel cell stack efficiently, without flooding and starvation problems.    DETAILED DESCRIPTION - The humidification system comprises a membrane humidifier (21) having at least one hollow fiber membrane in a housing (I), another membrane humidifier (22) having at least one hollow fiber membrane in a housing (II), and a flow path switching valve provided at the rear end of membrane humidifier (22) to selectively open and close a discharge flow path of humidified air to be supplied to fuel cell stack of fuel cell. The diameter of hollow fiber membrane of humidifier (22) is more than diameter of hollow fiber membrane of humidifier (21). The pore size of hollow fiber membrane of humidifier (22) is less than the pore size of hollow fiber membrane of humidifier (21). The membrane humidifiers (21,22) are arranged in parallel with respect to direction of flow paths through which dry air supplied from outside air is fed into membrane humidifiers (21,22) and flow paths. The dry air passes through a portion or whole of hollow fiber membranes and supersaturated humid air discharged from the fuel cell stack of fuel cell passes through housings (I,II) to carry out moisture exchange between dry air and supersaturated humid air and produce humidified air.    DESCRIPTION OF DRAWING(S) - The drawing shows the use of membrane humidifiers having hollow fiber membrane bundles in the humidification system.    Membrane humidifiers (21,22)    Hollow fiber membranes (21a,22a)