To restrain performance degradation of a fuel cell due to carbon poisoning at a cathode side, through alleviation of unevenness of hydrogen density inside an anode at start-up, as well as restraint of degradation of a power storage part, in a fuel cell system.

The system is provided with a fuel gas supply valve 42, a hydrogen pump 48 fitted to a fuel gas circulation flow channel 46, a secondary battery 50 supplying power to the hydrogen pump 48, and a control part 68. The control part 68 is provided with a fuel gas density estimation means estimating hydrogen density in a flow channel where hydrogen gas or hydrogen offgas flows, and a fuel gas supply valve pump control means. The fuel gas supply valve pump control means sets a target revolution of the hydrogen pump 48 from a detection value of a charged volume of the secondary battery 50 and an estimated value of the hydrogen density, and controls so as to open the fuel gas supply valve 42 after a given elapse of time from start of rotation of the hydrogen pump 48 by the target revolution.

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