To realize a small capacitance by digging down portions existing between electrode fingers from a portion where the electrode fingers are provided.

A portion 10b between electrode fingers 12 is dug down just for t2 in respect to a portion 10b where the electrode fingers 12 are formed. The t2 greater than '0' and formed by a technique such as reactive ion etching(RIE). By digging down the portion 10b between these electrode fingers 12, reducing action is generated in the value of an effective dielectric constant related to electrostatic coupling of the mutually adjacent electrode fingers 12. Namely, among the electric lines of force generated from an arbitrary electrode finger 12, the line passed through a piezoelectric substrate 10 is passes through the area of dielectric constant = 1, namely, in the air and the effective dielectric constant related to electrostatic coupling between the electrode fingers 12 is lowered. As a result, even without extremely reducing the number of electrode fingers or opening length, electrostatic coupling between two terminals can be made into small value.