1. An information retrieval apparatus which retrieves information that meets a search request, from a database which stores information to be retrieved, said apparatus comprising: a general association dictionary storage unit operable to store a relationship between words included in the information stored in the database; a preference-based association dictionary storage unit operable to store, together with an association value, a relationship between words which reflect user's preferences regarding the information stored in the database, the association value being a value indicating a degree of association between the words; a preference related word obtainment unit operable to obtain, as a related word, a word whose association value with respect to inputted information indicates a predetermined threshold or greater, from said preference-based association dictionary storage unit, the inputted information being information inputted by the user, and the related word being a word relating to the inputted information, and decrease the predetermined threshold by a predetermined amount when receiving a search condition expansion command for requesting an expansion of a search condition; an unobtained related word judgment unit operable to judge whether or not an unobtained related word is obtained by said preference related word obtainment unit, in a case where the search condition expansion command is inputted, the unobtained related word being related to the inputted information; a general related-word obtainment unit operable to obtain the related word from said general association dictionary storage unit in a case where said unobtained related word judgment unit judges that the unobtained related word is not obtained; a search condition generation unit operable to generate a search condition using the unobtained related word, in a case where said unobtained related word judgment unit judges that the unobtained related word is obtained, and generate a search condition using the related word obtained by said general related word obtainment unit, in the case where said unobtained related word judgment unit judges that the unobtained related word is not obtained; and a retrieval unit operable to retrieve, from the database, information which meets the search condition generated by said search condition generation unit.