1. A non-dialogue-based learning apparatus, comprising: a speech input unit for inputting speeches; a speech recognition unit for recognizing the input speech; and a behavior and dialogue controller for controlling behaviors and dialogues according to speech recognition results, wherein the behavior and dialogue controller has a topic recognition expert to memorise contents of utterances and to retrieve the topic that best matches the speech recognition results, and a mode switching expert to control mode switching, wherein the mode switching expert switches operation modes of the apparatus between a learning mode and an execution mode in accordance with a command in the recognized speech input through a user utterance, wherein in the learning mode, the topic recognition expert creates a word graph from each speech that the user utters as corresponding to a new topic, the word graph being a plurality of candidate words or sentences that are constructed by words contained in a predetermined dictionary together with associated matching probabilities with respect to the uttered speech, and the topic recognition expert registers each of words constituting the word graph together with associated occurrence frequencies as representing said new topic, the topic recognition expert registering, as representing said new topic, words and associated occurrence frequencies generated by a plurality of the word graphs when the user utters plural speeches as corresponding to the new topic, and wherein in the execution mode, the topic recognition expert performs searches from among topics that have been registered in the learning mode, and selects the maximum likelihood topic.