NOVELTY - The system has a circulation pump (3A) with a main portion (30) having an entrance (31) connected to hot water storage tank, and an exit (32) connected to heat source, a first pump chamber connected with entrance, and second pump chamber connected with exit. A drive mechanism (40) has two diaphragms (41,42) that respectively reciprocate in first and second pump chambers so that pump operation cycle becomes mutual with antiphase. A path portion (50) connects between pump chambers, and enables liquid communication from first pump chamber to second pump chamber.    USE - Hot-water supply system for pump apparatus such as water circulation pump.    ADVANTAGE - Since the second pump chamber is connected with the first pump chamber through the path portion, hydraulic pressure in the hot water storage tank transduced into the entrance acts on first and second diaphragms simultaneously. Therefore, since a portion for a water pressure variation is canceled with pressure-receiving area difference between both diaphragms even if fluctuate sharply the hydraulic pressure in hot water storage tank, the fluctuation of the load of drive mechanism is suppressed. Since the fluctuation of the load added to drive mechanism is suppressed according to the circulation pump even if the water pressure variation in hot water storage tank arises, while a stable discharge flow volume is maintainable, the power consumption of circulation pump is also reduced. Moreover, since the stress added to the drive mechanism is suppressed even if excessive hydraulic pressure is input, a water-pressure-resistant characteristic is improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view illustrating the structure of the pump apparatus. (Drawing includes non-English language text)    Circulation pump (3A)    Main portion of circulation pump (30)    Entrance of circulation pump (31)    Exit of circulation pump (32)    Drive mechanism (40)    Diaphragms (41,42)    Path portion of circulation pump (50)