PURPOSE: To detect signal energy, generated from a strand wire defective part, without a propagation loss by forming an electric wire clamping surface of a piezoelectric element mounting jig almost equal to a contour line of external shape of a strand wire but forming the electric wire clamping surface to a groove of spiral shape matched with a twisting pitch of the strand wire.

CONSTITUTION: Two divided holders 5, molded almost equally to a contour line of external shape of a strand wire 1, are mounted to the strand wire 1. Next, these holders 5 are tightened to specified torque or more by screws 3. Here, it is preferable that grease 6 is left as applied between the strand wire 1 and the holder 5. Thus by mounting the holder 5 to the strand wire 1, the holder 5 is brought into uniform contact with each element wire to increase also a contact area, so that signal energy from a defective part can be all extracted to substantially decrease a signal propagation loss between the element wire and the holder 5.