1. A category classification processing device comprising: an example storage unit configured to store Q&A (questions and answers) examples including pairs of questions and answers; a log storage unit configured to store search keyword logs relevant to searching the Q&A examples; and a processor configured to perform a search process and a category extraction process, wherein the search process includes sending, to a terminal device, an input screen image used for inputting one or more keywords, searching the example storage unit using the one or more keywords input to the input screen image to extract Q&A examples based on the one or more keywords, sending identification information items expressing the extracted Q&A examples to the terminal device, and storing, in the log storage unit, search keyword logs including the identification information items relevant to the Q&A examples which are referred to at the terminal device using the identification information items, the identification information items in the search keyword logs being associated with the one or more keywords input to the input screen image during the searching, and wherein the category extraction process includes aggregating keyword storage frequencies expressing a number of times each of the keywords input to the input screen image during the searching and recorded together with the Q&A examples in the search keyword logs, is stored for each of the Q&A examples, and calculating keyword group storage frequencies expressing a number of times each of keyword groups is stored for each Q&A example, each of the keyword groups including n keywords recorded together with the Q&A examples in the search keyword log, where n is a positive integer, extracting, as category candidates of each of the Q&A examples, m top keywords in a descending order of the keyword storage frequencies, where m is a positive integer, and extracting, as tentative sub category candidates of each Q&A example, keywords belonging to m top keyword groups in the descending order of the keyword group storage frequency, using the extracted category candidates as categories, and using the extracted tentative sub category candidates as sub categories, and associating the categories with the sub categories, and associating the sub categories with the Q&A examples, to store the categories, the sub categories, and the Q&A examples that are associated with each other in the example storage unit.