Abstract: A fuel cell system that can realize generation of electrical energy with improved energy efficiency as well as with a simplified construction of the system, and a hydrogen supply device used in the fuel cell system. The fuel cell system 1 comprises a fuel cell unit 4 using hydrogen as a fuel, and a hydrogen supply device 3 to supply hydrogen gas to the fuel cell unit 4. The hydrogen supply device 3 comprises a fuel-side electrode 8 to decompose a fuel whose standard oxidation-reduction potential is equal to or less than zero, a hydrogen-production-side electrode 9 for producing hydrogen, and an electrolyte membrane 10 interposed between them. This hydrogen supply device 3 can promote a spontaneous electrolytic reaction of the fuel whose standard oxidation-reduction potential is equal to or less than zero.