It is often required to extract the sound of an objective instrument played in concert with other instruments. Microphone array is one of the effective ways to enhance a sound from a specific direction. However it is not effective in an echoic room such as concert hall. The pickup microphone attached on the specific musical instrument is often employed to obtain the sound exclusively from other instrumental sounds. The obtained timbre differ from the one we hear at the usual listening position. The purpose of this paper is to propose a new method of sound separation that utilizes the piezoelectric device attached on the body of the instrument. The signal from the attached device has a different spectrum from the sound heard by the audience but has the same frequency components as the instrumental sound. Our idea is to use the device signal as a modifier of the sound focusing filter applied to the microphone sound at the listening position. The proposed method firstly estimates the frequency components of the signal from the piezoelectric device. The frequency characteristics for filtering the microphone sound are changed so that it pass the estimated frequency components. Thus we can extractthe target sound without distortion. The proposed method is a sort of dynamic sparseness approach. It was found that SNR is improved by 8.7dB through the experiments.