A resonance layer (30) and an acoustic separation layer (34) are arranged adjacent to each other between a piezoelectric element (24) and a circuit board (16) provided with an electronic circuit for driving the piezoelectric element. The acoustic impedance of the resonance layer (30) is higher than that of the piezoelectric element (24), and the acoustic impedance of the acoustic separation layer (34) is lower than that of the circuit board (16). An ultrasonic wave is reflected at the interface between the resonance layer (30) and the acoustic separation layer (34) where the difference in acoustic impedance is large, and the ultrasonic wave propagating to the circuit-board (16) side is reduced.
