1. A signal processing apparatus comprising: a processor; and a memory to store computer-executable instructions which, when executed, cause the processor to: segment a signal feature quantity, which corresponds to a feature of a content signal comprising audio and/or video, into segments having a given time length; cluster the segments into classes on the basis of similarity among signal feature quantities of the respective segments; generate existence probability functions indicating probabilities that respective segments exist with respect to an elapsed time period of the content signal for segments belonging to the respective classes; calculate similarities among the existence probability functions for the respective classes; obtain integrated classes by integrating classes having existence probability functions with a level of similarity greater than a threshold similarity level; and output identification information of the integrated classes to which the respective segments belong.