1. A signal processing apparatus comprising: amplitude extraction means for determining an amplitude of an input audio signal corresponding to each of a plurality of outputs of a spectral decomposition unit comprised of a plurality of bandpass filters or an FFF unit providing similar output, the audio signal having a plurality of contents in a time-division manner; and change-point detection means for detecting a change point of content in the input audio signal based on the temporal change of the amplitude extracted by said amplitude extraction means.