PURPOSE: To eliminate the lead wires connecting a string vibration detecting unit and a circuit board and to simplify wirings as well as to decrease incoming noises by providing the above-mentioned detecting means on the circuit board disposed on a tremolo unit.

CONSTITUTION: The mechanical vibrations of a string 18 are transmitted via a holder 34 and a string height adjusting screw 38 to a piezoelectric element 39 and generates the potential difference corresponding to the vibrations between a front surface B and a rear surface A. The potential of the surface B drops to the ground via a holder 34, an octave adjusting screw 22, a bridge base 15, and a ground wire 33. The electric signal of the surface A is inputted via planar metallic foil 28 and wire-shaped metallic foil 29 to a circuit section where the signal is amplified by a preamplifier 26. The amplified signal is outputted via an output line 32 to an amplifier on the outside of a guitar body. Then, the need for the lead wires for inputting to the circuit section is eliminated and the complicating of the wires by drawing around the wires in the body 11 is obviated. In addition, the trouble that the sound quality is degraded by picking up the incoming noises is eliminated.