To decide a relation between the phase of a reflected wave and the phase of an excitation wave by deciding a central distance between a positive electrode finger and a negative electrode finger, a central distance between the negative electrode finger and a floating electrode finger and a central distance between the floating electrode finger and a next positive electrode finger to be prescribed values.

A constitution in which the negative electrode finger 1 of electrode finger width λ/8, a positive electrode finger 2 of electrode finger width λ/8 and the floating electrode finger 3 of electrode finger width λ/8, which is not brought into contact with the positive/negative electrode fingers, are sequentially arranged in the transmission direction of a surface wave is periodically repeated as a basic unit. Unidirectional electrode is constituted in such a way that the central distance of the electrodes 1 and 2 is λ/4, that between 2 and 3 is λ/4, and that between 3 and 1 (1') of the next period is λ/2, respectively. The unidirectional electrode is arranged on a piezoelectric substrate 100. Thus, a minimum electrode finger width is λ/8 and the unidirectional electrode can be constituted. The phase of reflection becomes a common mode to one side but becomes completely opposite to the phase of excitation.