PURPOSE: To provide an ultrasonic transducer in which an ultrasonic device composed of a piezoelectric thin plate and an interdigital electrode is provided on a non-piezoelectric board.

CONSTITUTION: In cases where an interdigital electrode 1 is used as an input electrode, when an electric signal is inputted to the interdigital electrode 1, only an electric-signal component thereof having a center frequency corresponding to the electrode 1 and frequencies therearound is converted to an elastic surface wave and propagates through a piezoelectric ceramics thin plate 2 at a velocity Vs. The portion of this elastic wave which satisfies the relationship wherein the velocity VS is greater than the velocity VCT of a transversal wave in a glass board 3 as a simple substance is converted to a bulk wave having a velocity VC to be leaked into the glass board 3. This bulk wave is converted to a longitudinal wave with a velocity VW at an interface between the glass board 3 and water to be radiated into the water.