Degradation of acetaldehyde using natural clay mineral, allophane was investigated in the dark and during UV irradiation in order to confirm its catalytic activity. A slight amount of acetaldehyde in gas phase was degraded and converted to acetic acid or carbon dioxide and water in the dark. The degradation was significantly promoted by the UV irradiation. Allophane should absorb UV light and convert its energy to heat energy because it is not a photocatalyst. The only surface of the allophane particle reached a high temperature. The acetaldehyde adsorbed on the allophane particle was degraded due to its catalytic activity.