NOVELTY - A water tank stores recovered water from condensed vapor from combustion exhaust gas and modifier of fuel cell. The water is used as source of water vapor for steam reforming. A waste water return line refluxes waste water ejected from an electric deionization apparatus, via overflow piping to recovered water tank. The fuel cell is equipped with deionizing apparatus, which comprises piping for pure water supply.    USE - For recovering condensed water from water vapor in combustion exhaust gas from devices such as modifier, for effective utilization of resources, in fuel cell power generating device.    ADVANTAGE - The waste water ejected from electric deionization apparatus is refluxed to a recovered water tank. The control unit switches the operation of deionization apparatus and the ion exchange water purifier intermittently, when water quality reduces and conductivity increases. Effective usage of concentrated waste water from deionizing apparatus is enabled and a self-sustaining fuel cell which replenishes water when recovered water runs short, is offered. The water-treating unit has inexpensive operation and maintenance.    DETAILED DESCRIPTION - A water tank (7) stores recovered water from condensed vapor from combustion exhaust gas and modifier (2) of fuel cell (1). The modifier supplies hydrogen rich reformed gas (by steam reforming of a hot hydrocarbon gas) with an oxidizing agent gas to fuel cell. The recovered water is used as source of water vapor for steam reforming. A concentrated waste water return line (52) refluxes the waste water ejected from an electric deionization apparatus (10), via overflow piping (8) to the recovered water tank, in the water-treating unit, for treatment. The fuel cell power generating device is equipped with deionizing apparatus, which comprises piping for pure water supply.    An INDEPENDENT CLAIM is also included for the operating method of water-treating unit for fuel cell power generating device. The operation of the electric deionizing apparatus is stopped and circulation of pure water is performed by operation of an ion exchange apparatus.    DESCRIPTION OF DRAWING(S) - The figure shows the explanatory block diagram of water-treating unit for fuel cell power generating device. (Drawing includes non-English language text).    Fuel cell (1)    Modifier (2)    Steam separator (5)    Condenser (6)    Recovered water tank (7)    Overflow piping (8)    Deionization apparatus (10)    Ion exchange water purifier (12)    Conductivity meter (13)    Switching valve (14)    Recovered water introduction piping (50)    Pure water extraction piping (51)    Concentrated waste water return line (53)