To make phase followup ability better and to save the space of the whole device by controlling the number of revolution of a servo motor according to the detection signal by a detection means and composing the phase of the exciting wave of a damping means to be modulated.

In an acceleration detector 3, the vibration of a structure is converted into an electric signal and a sine-wave shaped signal is formed by a band pass filter 9 to take out a prescribed frequency. After the formed input signal is made into digital data by an A/D converter 10, the digital data is inputted to a phase controller 11, a phase modulation is performed for the exciting wave of a balancer 2 for damping in the controller 11 and the vibration frequency of the structure is recognized. After a pulse signal is prepared by a pulse generator 12 based on the recognized vibration frequency and the pulse signal is amplified by an amplifier 13, a servo motor 4 is driven so as to operate the balancer 2 by the same frequency as the structure.