1. An object classification apparatus comprising: a speech/image data storage unit configured to store a spoken sound of a name of an object and an image of the object; a speech confidence level calculation unit configured to calculate a speech confidence level of a speech for the name of the object with reference to a speech model of a name of a known object, the speech confidence level being a ratio of speech likelihood of the name of the object for the speech model of the name of the known object to the highest speech likelihood among speech likelihoods calculated in phoneme sequence for the spoken sound of the name of the object; an image confidence level calculation unit configured to calculate an image confidence level of an image of an object with reference to an image model of a known object, the image confidence level being a ratio of image likelihood of the object for the image model of the known object to the highest image likelihood among image likelihoods that an image model of the known object may take; and an object classification unit configured to compare an evaluation value, which is a combination of the speech confidence level and the image confidence level, with a threshold value, and to classify a target object into an object group determined according to whether the spoken sound of the name and the image are known or unknown.