PURPOSE: To make backing effect constant at the entire surface of a piezoelectric element, by making the thickness of a bonding layer between the piezoelectric element and a backing material constant, or providing a state where the bonding layer is equivalently absent.

CONSTITUTION: The back surface of a piezoelectric element 10 is constituted by a thin film, whose thickness is a 1/4 wavelength of generated ultrasonic wave, and sound absorbing material 12. The sound impedance of the thin film 11 is made larger than that of the sound absorbing material 12. Therefore, backing effect for the piezoelectric element becomes uniform, and the dispersion in sound conversion efficiency can be confined in specified values. Since the backing material is constituted by the thin film, whose sound impedance is, e.g., 3×106W 5×106kg/m2.sec and the sound absorbing material, whose sound impedance is 1.5×106W2×106kg/m2.sec, the range of selection of materials can be expanded.