1. A keyword acquiring device, comprising: a tree storage unit that stores a keyword tree obtained by performing hierarchization according to the number of appearances and relevance in advance using a keyword associated with content as a node; a log storage unit that records a keyword associated with content previously selected by a user as a log; a partial tree extracting unit that extracts a plurality of keywords based on a predetermined condition from a log corresponding to a certain user recorded in the log storage unit, and extracts a partial tree including the extracted keywords from the keyword tree; and a keyword acquiring unit that acquires a keyword in a node having no other lower node as a keyword related to the user's interest.