PURPOSE: To attain efficiently design of the titled element by utilizing an photo-acoustic effect to generate a surface acoustic wave equivalent to the surface acoustic wave produced by an interdigital electrode of an optional shape and detecting the surface acoustic wave by a detecting with the interdigital electrode whose frequency characteristic is known.

CONSTITUTION: A laser beam subject to pulse intensity modulation is radiated to a surface acoustic element 11. Then a surface acoustic wave and a bulk wave are generated in the element by taking a radiation point as a wave source position based on the photo-acoustic effect. Only the surface acoustic wave only is detected by an interdigital electrode 15 whose frequency characteristic Ho is known. A detected electric signal is amplified by an amplification circuit 16, and sent to a signal amplitude and phase difference detection circuit 17. The amplitude of the electric signal is divided (normalized) by an amplification factor proportional to a laser light quantity fed from an amplifier circuit 9. Simultaneously, a phase difference between a reference sinusoidal wave signal fed from a reference signal generating circuit 12 and an electric signal to be detected is measured. The surface acoustic wave element 11 is fixed to a biaxis stage 19, which controls the laser beam radiating position accurately via a stage driving circuit 20.