Environment-friendly anodizing of aluminum was developed. This method used high-pressure carbonic acid aqueous solution as an electrolyte solution. The anodic oxide film was formed under conditions that the anodizing voltage was 50-250 V, the electrolytic temperature 10-70 degrees C, the electrolytic pressure 0.1-30 MPa and the electrolyzing time 10-600 min. The thickness of the oxide film increased with the increase in the anodizing voltage, electrolytic temperature, electrolytic pressure and the electrolyzing time. It was around 0.1-2.0 mu m. The growth ratio of the barrier layer, which is defined by dividing the thickness of the barrier layer by the anodizing voltage, was 1.1 nm/V. The ratio of the average cell size to the anodizing voltage was 2.1 nm/V. These values were close to those of oxide films of aluminum produced in a conventional electrolytic bath such as the sulfuric acid bath.