A dense thick film of mixed-conductive perovskite-type oxide (La0.6Sr0.4Co0.8Fe0.2O3) was prepared by means of a slip-casting technique. Film thickness could be easily controlled by changing the casting time. Oxygen semipermeability through the film increased with decreasing film thickness. An acid treatment of the film was found greatly to improve the oxygen semipermeability, because the treatment could remove surface impurities like SrO.