PURPOSE: To prevent deposition of potassium carbonate on the electrode surface caused by carbon dioxide by letting air which is supplied to a fuel cell pass in a carbon dioxide remover and letting it pass a vapor section on the electrolyte surface in an electrolyte tank to add moisture.

CONSTITUTION: A fuel cell 1 is operated by supplying air which serves as oxidizing gas and hydrogen which serves as fuel gas and by circulating electrolyte 6 in an electrolyte tank 5. Air A is let to pass in a carbon dioxide remover 4 filled with soda lime 3 which is carbon dioxide absorber with a blower 2, and made to flow in zigzags in a vapor section in an electrolyte tank 5 in which a multiple row of separating plates 9 are arranged to bring into contact it with vapor generated from the surface of electrolyte 6 to add moisture, then supplied to the cell 1. By this simple process, deposition of potassium carbonate caused by carbon dioxide in the air is prevented, and gas diffusion is increased. Therefore, a fuel cell having long life is provided.