Information retrieval apparatus, information retrieval method and navigation apparatus with information retrieval function

An information retrieval apparatus for retrieving a target information corresponding to an input data, from an information group including a plurality of information, the plurality of information being divided into a plurality of levels of a hierarchical structure in the information group, characterized in that the information retrieval apparatus has: a first searching device for searching a registration area in which a plurality of information are registered for the target information; a first determining device for determining whether or not there is the target information in the registration area; a second searching device for searching the information group for the target information for each level, if the first determining device determines that there is not the target information in the registration area; a second determining device for determining whether or not there is the target information in the information group; a registering device for registering the target information into the registration area from the information group, if the second determining device determines that there is the target information in the information group; and an outputting device for outputting the target information from the registration area if the first determining device determines that there is the target information in the registration area, and for outputting the target information from the information group if the second determining device determines that there is the target information in the information group.