PURPOSE: To realize a proper moisture content of supplied air by appropriately heating an aqueous alkali solution in a counterflow type carbon-dioxide-absorbing tower in which the aqueous alkali solution is used as an absorbent for removing carbon dioxide contained in said supplied air.

CONSTITUTION: At first, air is introduced from an air supply pipe 10 into a caustic-alkali-solution storing tank 3. Caustic alkali solution contained in the tank 3 is heated with a heater 7, and is controlled to have a desired temperature by means of a temperature controller 9. The temperature of the solution is detected with a temperature detector 8 which is connected to the controller 9. Besides, the above heated caustic alkali solution is introduced into a spray nozzle 4 through a caustic-alkali-solution circulating pipe 6 by means of a pump 5, and sprayed down from the nozzle 4. As a result, air supplied from the pipe 10 can be heated to a temperature suitable for the condition of this invention by passing through such a carbon-dioxide-gas removing device (A). Here, air introduced into a fuel cell 2 is discharged from an air exhaust pipe 11.