To eliminate electric wiring for introducing a voltage generated in a piezoelectric member to a receiving circuit by detecting a pressure being applied to the piezoelectric member based on the variation in the optical characteristics of an electrooptical member subjected to a field generated from the piezoelectric member.

When an ultrasonic oscillator 1 (piezoelectric member) is subjected to an ultrasonic wave W, the oscillator 1 oscillates to generate a voltage between the electrodes 11, 12 through piezoelectric effect. Since an electrode 22 is grounded along with the electrode 11, a potential difference equivalent to the voltage generated in the oscillator 1 appears between the electrodes 22, 12 and a field is applied to an electrooptical element 2. Consequently, the refractive index is varied through the electrooptical characteristics of electrooptical element 2 depending on the field strength thus varying the effective optical path length of a light passing through an optical fiber 41, reflected on a reflective surface 21 and taken out through an optical fiber 42. Since the phase of light taken out through the optical fiber 42 is different from that of incident light due to variation of the optical path length, an optical signal phase modulated depending on the ultrasonic wave is obtained.