NOVELTY - The apparatus has an air line (31) connected from a cathode outlet of a fuel cell stack (20) to humid air inlets (15a, 15b) of a membrane humidifier (10) to supply humid air. Exhaust lines (33a, 33b) are connected to air outlets (17a, 17b). A flow control valve (32) is provided in the air line, and controls introduction of humid air into the humid air inlets. An exhaust valve (34) opens and closes flow paths of the exhaust lines. A controller controls the opening and closing of the flow control valve and the exhaust valve based on operating conditions of the fuel cell stack.    USE - Humidification apparatus for a fuel cell system of a vehicle for humidifying air using a membrane humidifier. Uses include but are not limited to a motor vehicle e.g. passenger automobile such as sports utility vehicle (SUV), bus, lorry and commercial vehicle, a watercraft such as boat and ship, an aircraft, a petroleum-powered or electric-powered hybrid vehicle, an electric vehicle, a plug-in hybrid electric vehicle, a hydrogen-powered vehicle and an alternative fuel vehicle.    ADVANTAGE - The membrane humidifier serves to supply moisture and heat contained in high-temperature gas discharged from the fuel cell stack to dry reaction gas at room temperature supplied to the fuel cell stack via the membrane surface, thus achieving humidification of the fuel cell stack and maintaining temperature of the fuel cell stack. The humidifier can control the flow of air discharged from the fuel cell stack through valve control process so as to reduce back pressure of the membrane humidifier at high flow rate and reduce the load of the air blower. The apparatus can control the flow of air in the counter-flow direction and the co-flow direction in the membrane humidifier by controlling the valves based on the operating conditions of the fuel cell stack so as to optimally control the amount of dry air to be humidified by the membrane humidifier, thus effectively preventing occurrence of flooding in the cathode of the fuel cell stack and avoiding performance reduction and local air starvation in the cathode.    DETAILED DESCRIPTION - The air after humidification is discharged through the air outlets. The flow control valve and the exhaust valve are designed as an electric 3-way valve. An INDEPENDENT CLAIM is also included for a non-transitory computer readable medium comprising a set of instructions for humidifying air using a membrane humidifier.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a humidification apparatus.    Membrane humidifier (10)    Humid air inlets (15a, 15b)    Air outlets (17a, 17b)    Fuel cell stack (20)    Air line (31)    Flow control valve (32)    Exhaust lines (33a, 33b)    Exhaust valve (34)