1. An interactive program search apparatus which searches out a program using a search word, said apparatus comprising: a program information storage unit configured to store program information indicating contents of programs on a per program basis; an association dictionary storage unit configured to store an association dictionary in which words included in the program information are stored in association with each other; an obtaining unit configured to obtain the search word; an obtainment history storage unit configured to store, as obtainment history information, the search word obtained by said obtaining unit and an obtainment frequency of the search word; a program search unit configured to search out programs from the program information using, as a search condition, the search word included in the obtainment history information; an association-source word extracting unit configured to extract an association-source word which is associated with an associated word, from among the words included in the obtainment history information; an associated word extracting unit configured to extract the associated word associated with the association-source word using the association dictionary; a reason word extracting unit configured to extract, as a reason word indicating a relationship between the association-source word and the associated word, a word included in the obtainment history information from among words each having (i) a degree of relevance equal to or greater than a predetermined degree of relevance with the association-source word and (ii) a degree of relevance equal to or greater than a predetermined degree of relevance with the associated word; and a display unit configured to display program search results which are the programs searched out by said program search unit, the associated word, and the reason word.