To investigate the frequency characteristic of a particular part of a seismic wave train, such as P, S and other prominent waves, it may be best to observe the filtered waves using filters of various frequency bands. This paper presents a method of obtaining a smoothed amplitude spectrum by means of analog filters. The procedure is quite simple and it is easy to treat a large volume of data by this method. The method is applied to several examples of seismic waves from microearthquakes and is found to be effective for distinguishing between various parts of the wave train in the frequency characteristic.