The allophane-titania composites were prepared on a macroporous filter. The allophane particles were dispersed in a titania sol, in which the filter was irradiated by ultrasonic waves, and then the resulting deposit on the filter was heated. During the photocatalytic degradation of trichloroethylene, the composite containing a small amount of allophane improved the adsorption ability of the titania and inhibited emission of the intermediate, phosgene. The appropriate UV irradiation intensity also inhibited the phosgene emission due to an effective adsorption-degradation relay.