NOVELTY - A circulating pump (17) comprises a drainage valve (27) for draining water out of a water collecting section (30). The drainage valve comprises a Pitot tube whose one end communicates with water collecting section by opening within water in water collecting section and whose other end communicates with drain port.    USE - For taking in a moisture mixture containing reaction produced water and non-reacted gas from fuel outlet port or an oxidant outlet port of a fuel cell and for refluxing the non-reacted gas to an inlet port of the gas of fuel cell in a fuel cell system (e.g. polymer electrolyte fuel cell system) used in a micro-gravity environment (claimed), such as the space, and closed environment on the ground, and useful in fields of using gas circulating apparatuses.    ADVANTAGE - The moisture mixture separating mechanism is capable of separating produced water in high purity. The moisture separating system can effectively conduct the moisture separation under the micro-gravity environment. The fuel cell system efficiently separates the non-reacted gas and the reaction produced water contained in the emission side in generating power by the fuel cell; returns only the non-reacted gas to the supply system; and effectively and actively uses the gas so that it contributes in power generation. The fuel cell system can be down-sized because the circulating pump and moisture separating mechanism can be integrated. Excellent effects such as improvement of performance of cells, reduction of operation cost, and realization of high power generation are achieved.    DETAILED DESCRIPTION - A circulating pump (17) comprises a pump casing having an intake port communicating with a lead-in passage for leading in the moisture mixture from a fuel outlet port or an oxidant outlet port of a fuel cell and a discharge port for sending the non-reacted gas to the inlet port of the gas of the fuel cell; impellers (25) disposed within the pump casing; a moisture trap (28) provided around the center of a front face of the impellers to rotate together with the impellers, for trapping moisture within the moisture mixture led into the pump casing via the intake port; a water collecting section (30) provided in communication with the moisture trap, for storing water drained out of the moisture trap by rotation of the moisture trap in a state of receiving centrifugal force; and a drainage valve (27) for draining water out of the water collecting section. The drainage valve comprises a Pitot tube whose one end communicates with water collecting section by opening within water in water collecting section and whose other end communicates with drain port. The moisture trap is composed of a gas transmitting porous material disposed to extend from a front part of impellers at an end of gas lead-in section and a rotary center part of impellers toward the outside in a radial direction. INDEPENDENT CLAIMS are included for:    (1) a drain valve; and    (2) a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing is a section view of a circulating pump.    Circulating pump (17)    Impellers (25)    Drainage valve (27)    Moisture trap (28)    Water collecting section (30)