Techniques provide for searching pieces of document data using a search keyword. The technique includes: calculating, as a first vector, respective first scores at which or respective probabilities that each of the pieces of document data belongs to clusters or classes; calculating, as a second vector, respective second scores at which or respective probabilities that the search keyword or a relevant keyword associated with the search keyword belongs to the clusters or the classes; calculating an inner product of each of the first vectors and the second vector, the calculated inner product being a third score of the corresponding piece of document data regarding the search keyword; and acquiring a correlation value from document data containing each keyword in a classification keyword set and document data with the third score that is equal to or more than a predetermined threshold or is included in a predetermined high-ranking proportion.