NOVELTY - A hydrogen exhaust system (40) comprises hydrogen exhaust line (41) provided at an anode outlet of a fuel cell stack (10); hydrogen purge valve (42); outlet line (41a) for exhausting purge hydrogen discharged by hydrogen purge valve; inlet line (43) for exhausting purge hydrogen to an air supply line; a vehicle state detector; controller for outputting a control signal for selecting either the cathode outlet or the cathode inlet of fuel cell stack as an exhaust path of purge hydrogen based on vehicle state; and valve (44) capable of being opened and closed by control signal of controller.    USE - Hydrogen exhaust system used for fuel cell motor vehicle (claimed). Uses include but are not limited to sports utility vehicles, buses, trucks, watercraft including boats and ships, aircraft, hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum).    ADVANTAGE - The system actively controls an exhaust path of hydrogen discharged from an anode of a fuel cell stack to maximize the safety of the vehicle by minimizing the possibility of explosion due to hydrogen exhaust and ensure the silence of the vehicle by minimizing noise generated during hydrogen exhaust. Since the water discharged from the fuel cell stack is suitably supplied to the air supply line of the fuel cell stack together with purge hydrogen discharged to the hydrogen exhaust line, it is possible to suitably increase the humidification performance of the fuel cell stack, thus suitably improving the performance and power of the fuel cell stack. While the anode exhaust gas of the fuel cell stack is suitably supplied to the cathode of the fuel cell stack to be oxidized, the purge hydrogen is suitably exhausted to the cathode outlet through the outlet line under certain conditions where the vehicle travels at a high speed or where the power of the fuel cell stack is high when the vehicle travels at a low speed, and thus it is possible to ensure stable operation of the fuel cell stack.    DETAILED DESCRIPTION - A hydrogen exhaust system (40) comprises:    (A) a hydrogen exhaust line (41) provided at an anode outlet of a fuel cell stack (10);    (B) a hydrogen purge valve (42) provided in the hydrogen exhaust line;    (C) an outlet line (41a) as a part of the hydrogen exhaust line at a rear end of the hydrogen purge valve, the outlet line exhausting purge hydrogen discharged by the hydrogen purge valve to an air exhaust line at a cathode outlet of the fuel cell stack;    (D) an inlet line (43) branched from the outlet line and exhausting purge hydrogen to an air supply line at a cathode inlet of the fuel cell stack;    (E) a vehicle state detector for detecting a vehicle state;    (F) a controller for outputting a control signal for selecting either the cathode outlet or the cathode inlet of the fuel cell stack as an exhaust path of purge hydrogen based on the vehicle state detected by the vehicle state detector; and    (G) a valve (44) capable of being opened and closed by a control signal of the controller to open one of flow paths of the inlet line and the outlet line such that purge hydrogen is discharged to either the cathode outlet or the cathode inlet of the fuel cell stack.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic flow diagram of a fuel cell system.    Fuel cell stack (10)    High-pressure regulator (23)    Low-pressure regulator (25)    Hydrogen exhaust system (40)    Hydrogen exhaust line (41)    Outlet line (41a)    Hydrogen purge valve (42)    Inlet line (43)    Valve (44)