PURPOSE: To simplify the constitution and to make the size of the filter small by combining a switched capacitor filter and a required filter preventing noise from the switched capacitor filter in a tracking filter for analyzing the vibration of an automobile whose frequency changes with number of revolutions of its engine.

CONSTITUTION: The cut-off frequency of a switched capacitor filter(SCF) 34 receiving an external clock via a frequency multiplication means 37 is controlled. Simultaneously, a frequency switching control means 38 receiving an output of the means 37 controls the cut-off frequency of an anti-aliasing filter(AF) 35 and a smoothing filter(SF) 36 interrupting aliasing noise and sampling noise respectively from the SCF 34 synchronously with the cut-off frequency of the SCF 34. Thus, lots of components such as operational amplifiers, resistors and capacitors in the tracking filter used to analyze a vibration electric signal representing the vibration of an automobile or the like are not required, the constitution is simplified and the size is made small.