PURPOSE: To maintain the performance of the sound wave transmitter-receiver in an expected state by fitting a piezoelectric element between a sound wave transmitter receiver fitting seat and a rack, and applying the piezoelectric element with vibration opposite in phase from the vibration of the rack.

CONSTITUTION: A feedback controller 6 applies a voltage to the piezoelectric element 1 according to data on the frequency and amplitude of the rack vibration outputted from the acceleration sensor 4 of the rack so that the opposite phase from the phase of the rack vibration is obtained, and then performs feedback control so as to minimize the output (fitting seat vibration) of the acceleration sensor 5 of the fitting seat. Namely, the fitting seat 3 (sound wave transmitter-receiver) is protected against the vibration of the rack 2 according to the amplitude A of the rack 2, the thickness amplitude B of the piezoelectric element 1, the amplitude C of the fitting seat 3, the vibration target value G of the fitting seat 3, and the control voltage H of the piezoelectric element 1. In this case, the piezoelectric element 1 is driven with the control voltage and vibrated B at the same frequency with and in the opposite phase from the vibration A of the rack 2, and both the vibration A and vibration B cancel each other to minimize the vibration C of the fitting seat 3.