To increase volumetric energy density of the whole system by housing a volume variable methanol water tank in a drain separating tank or a vessel arranged in an oxidizing agent side outlet of a fuel cell.

Methanol water is supplied to a methanol reforming-reformed gas refining system 34 through a pump 33 from a volume variable methanol water tank 32 housed in a drain separating tank 37 arranged in an oxidizing agent side outlet of a fuel cell 31, and high purity fuel hydrogen is obtained. This fuel hydrogen is humidified by a humidifier 33, and is inputted to the fuel cell 31. On the other hand, an oxidizing agent is inputted to the fuel cell 31 after being humidified by a humidifier 38. A residual oxidizing agent is introduced to the drain separating tank 37 together with moisture generated by fuel cell reaction and moisture moved to the cathode electrode side together with a hydrogen ion, and gas and liquid are separated from each other.