NOVELTY - The system has a compressor (12) sucking and pressurizing oxidant gas i.e. air, to discharge the gas toward a fuel cell. A water injection valve (13) injects water toward a suction port of the compressor. An oxidant gas return passage i.e. pipe (24), connects an upstream portion of an oxidant gas supply passage i.e. pipe (20), in an upstream side of the compressor and a downstream portion of the oxidant-gas supply passage in a downstream side of the compressor to bypass the compressor. A regulation valve i.e. regulator (44), is provided in the return passage and provided with an adjustable opening.    USE - Fuel cell system for an automobile.    ADVANTAGE - The system has the water injection valve injecting water into the oxidant gas, thus suppressing back-flow and leakage from rotor clearance even in low air volume area of the compressor, and hence enhancing air supply efficiency of the compressor. The system separates and returns a part of the oxidant gas to the upstream side of the compressor and passes the part of the oxidant gas through the compressor again such that temperature of discharged gas is increased, and saturated vapor amount is increased to sufficiently and properly humidify the oxidant gas, thus avoiding the need for a humidifier, and hence reducing size of the system. The water contained in exhaust oxidant gas can be supplied to the water injection valve and recycled to be used in the oxidant gas, thus avoiding the need for an external water supply pipe for humidification purpose. The system has a hydrometer for measuring humidity of the oxidant gas to be supplied to the fuel cell to control humidification amount based on the measured humidity, thus ensuring stable operation of the fuel cell, and hence applying the system for the automobile requiring high reliability.    DESCRIPTION OF DRAWING(S) - The drawing shows a hydraulic circuit block diagram of a fuel cell system.    Fuel cell stack (10)    Compressor (12)    Water injection valve (13)    Pipes (20, 24)    Regulator (44)