1. An information retrieval apparatus by which a user retrieves information, comprising: usage-information acquisition unit acquiring a set of pieces of usage information that is information used by the user in past and manners of usage of the respective pieces of usage information; and user's preference tendency extraction unit extracting a preference of the user from the set of pieces of usage information and the manners of usage acquired by the usage-information acquisition unit, wherein: the user's preference tendency extraction unit extracts a preference tendency of the user by: obtaining a degree of importance that is a degree at which the user puts importance on the usage information on the basis of the manners of usage of the respective pieces of usage information; assigning weights to retrieval keys included in the usage information on the basis of the obtained degree of importance; and obtaining sum of weights of retrieval keys common among the respective pieces of usage information in the set of pieces of usage information.