PURPOSE: To bring the attenuation of a reflected wave due to surface ruggedness of a piezoelectric film to be 45dB or over by satisfying a specific relation among a width of a finger electrode and an interval between adjacent finger electrodes.

CONSTITUTION: An interdigital transducer 14 is formed on one major face of a glass substrate 12 at both sides in its lengthwise direction respectively and the interdigital transactor 14 has plural finger electrodes 16 and a piezoelectric film 18 is formed to cover the interdigital transducer 14 on the glass substrate 12. In this case, the relation of 2≤l/s is satisfied, where (s) is the width of the finger electrode 16 and (l) is the interval between adjacent finger electrodes. Thus, the attenuation of an undesired reflecting wave due to surface ruggedness of the piezoelectric film 18 is suppressed to a practical value being 45dB or over.