An information retrieval scheme using classification trees with respect to a plurality of information sources connected to a network is disclosed, where each classification tree having a hierarchical structure defined in terms of entries. In the information retrieval apparatus, classification tree class information for each classification tree class defined among entries constituting each classification tree is managed, where each classification tree class information indicating retrieval condition parameters for each classification tree class. Then, the information retrieval is controlled by receiving from a user a retrieval request indicating a retrieval target entry that was obtained by the user by tracing the classification trees, generating retrieval commands with respect to selected information sources among said plurality of information sources by referring to each classification tree class information corresponding to the retrieval target entry that is managed by the class definition unit, obtaining a retrieval result of a retrieval with respect to the selected information sources using the retrieval commands, and returning the retrieval result to the user.