PURPOSE: To improve sound pressure characteristics by forming a projection pattern which contacts a piston vibrator on one end surface of a contactor.

CONSTITUTION: The piston vibrator 1 which has an electrode formed over the entire surface is put in the hoe 4 of a storage member 3 together with a damper 2 and its surface is covered with a front surface plate 5 which is a quarter as thick as the characteristic wavelength of the vibrator 1. The contactor 6 which is formed of silicone rubber, etc., cylindrically has the projection pattern 7 which contacts the vibration 1 and improves sound pressure characteristics. Therefore, only an effective part of an ultrasonic wave transmitted by the vibrator 1 over the entire surface is transmitted to the lower end surface of the contactor 6 through the projection pattern 7 and sound pressure on the orientation axis of a sound field produced as a result has no complicate disorder in a short-distance sound field, thereby improving sound pressure characteristics.