Recently, considerable attention of material scientists and mechanical engineers has been devoted to measurement based on atomic force microscopy (AFM). Self-excitation is known to be an effective excitation method for AFM probe to measure the surface of a biological molecule in liquid. For practical use of a probe in liquid, we must realize a self-sensing and self-actuating AFM probe using PZT instead of using a conventional optical lever method. However, frequency characteristics of the PZT are very complex in applications for probe behavior measurement. For detecting sensor characteristics, the dynamics of the cantilever to which the PZT is attached are extracted from the PZT sensor output signal. To this end, we examine the frequency response of the PZT output signal in the case where the cantilever is excited with constant response amplitude using a PZT actuator. Then, we establish a method to process the signal so that the frequency characteristic of the PZT sensor has no high gain for the frequency range other than the first natural frequency. Finally, we conduct experiments to verify that the resultant signal is suitable to generate van der Pol-type self-excited oscillation.