A ceramic honeycomb filter including trapping holes forming lattices is used to detect particulates trapped in the filter. An electromagnetic wave is transmitted to the filter in a plane perpendicular to the longitudinal direction of the trapping hole. The electromagnetic wave after the electromagnetic wave passes through the filter is received and the particulates trapped in the filter are detected on the base of a received intensity of the electromagnetic wave. The electromagnetic wave has a frequency of 0.294 c/a or more and c/a or less (a represents a lattice spacing of said trapping holes and c represents the speed of light). The electromagnetic wave is irradiated to the filter wherein the particulates are not trapped and an inclined angle θ of the electromagnetic wave with respect to the lattices is changed so as to increase a received intensity of the electromagnetic wave.