A glass-supported pure anatase film with an extremely high photocatalytic activity has been prepared. Photocatalytic properties of the film were investigated by measuring the degradation of gaseous acetaldehyde under relatively strong (black fluorescent light) and weak (white fluorescent light) UV light irradiations. It was found that the photocatalytic efficiency was higher than that of Degussa P25, one of the most efficient commercial TiO2 powders.