NOVELTY - Fuel cell system comprises fuel cell to generate electric power using fuel gas supplied to anode and oxidant gas supplied to cathode; circulation path through which off-gas discharged from fuel cell is returned to fuel cell again; water reservoir disposed in circulation path and stores water separated from off-gas; water level detector for detecting or estimating level of water stored in water reservoir; water discharger to discharge water stored in water reservoir; and controller.    USE - Fuel cell system.    ADVANTAGE - The water stored in water reservoir is refluxed to fuel cell along with gas flow and fuel cell comes into moisture excessive state when fuel cell system is started up after being stopped, thus ensuring stability at startup of fuel cell system while ensuring stable operation.    DETAILED DESCRIPTION - Fuel cell system (1) comprises fuel cell (10) to generate electric power using fuel gas supplied to anode and oxidant gas supplied to cathode; circulation path through which off-gas discharged from fuel cell is returned to fuel cell again; water reservoir disposed in circulation path and stores water separated from off-gas; water level detector for detecting or estimating level of water stored in water reservoir; water discharger to discharge water stored in water reservoir; and controller (51), in which during monitoring after stop of fuel cell system or at startup of fuel cell system, controller operates water discharger to discharge water stored in water reservoir when controller determines that level of water detected or estimated by water level detector is equal to or higher than predetermined reference water level. The controller includes inclination angle acquiring device for acquiring inclination angle of moving body equipped with fuel cell system. It executes determination regarding water level by comparing reference water levels after corrections depending on inclination angle and level of water detected or estimated by water level detector.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a fuel cell system.    Fuel cell system (1)    Fuel cell (10)    Hydrogen tank (21)    Ejector (23)    Air compressor (31)    Back pressure control valve (32)    Radiator (41)    Cooling water pump (42)    Controller (51)    Inclination sensor (52)