When input electric signals E.sub.i (i=1, 2, . . . , n) with frequencies f.sub.i (i=1, 2, . . . , n), respectively, are applied to an input interdigital transducer, elastic waves are excited in a piezoelectric substrate. Leaky components of the elastic waves are radiated into a material in the form of longitudinal waves, which are reflected at a bottom boundary-surface of the material. One of reflected longitudinal waves are detected at an output interdigital transducer as a delayed output signal with one of the frequencies f.sub.i. When the delayed output signal is applied to at least one interdigital transducer for radiation, an elastic wave is excited in the piezoelectric substrate. A leaky component of the elastic wave is radiated into the material in the form of a longitudinal wave, which is reflected at the bottom boundary-surface and re-reflected at a top boundary-surface of the material, and thus a continuous reflection occurs.