NOVELTY - Fuel cell system ion exchanger for passing water through it, comprises an apparatus body having internal filters at upper and lower positions, ion exchange resin filling a space between the filters; a water supply port provided at a lower position of the apparatus body, for supplying the water into the ion exchange resin; a water discharge port; an air container provided at an upper position of the apparatus body; and an electric conductivity measuring unit provided in the air container at a position above the water discharge port.    USE - The fuel cell system ion exchanger is useful for passing water through it; and useful for solid oxide fuel cell for removing impurities contained in at least water used for steam reforming (all claimed).    ADVANTAGE - The fuel cell system ion exchanger makes it possible to achieve improvement in the ion exchange efficiency, improvement in the durability, reduction in the number of components, and reduction in the number of steps of maintenance operation.    DETAILED DESCRIPTION - Fuel cell system ion exchanger for passing water through it, the water contained in an exhaust gas discharged from a fuel cell for generating electricity by electrochemical reactions of a fuel gas and an oxygen-containing gas, comprises an apparatus body having internal filters at upper and lower positions, ion exchange resin filling a space between the filters; a water supply port provided at a lower position of the apparatus body, for supplying the water into the ion exchange resin; a water discharge port provided at an upper position of the apparatus body, for discharging the water which has passed through the ion exchange resin; an air container provided at an upper position of the apparatus body, for containing the air mixed into the apparatus body in a concentrated manner; and an electric conductivity measuring unit provided in the air container at a position above the water discharge port, for measuring electric conductivity of the water which passed through the ion exchange resin.