There is provided a dictionary creation device and dictionary creation method which optimally creates and updates a dictionary for classifying, searching, or extracting text information in accordance with a changes in content of text information groups. The dictionary creation device includes: a keyword extraction unit that extracts a keyword from inputted text information; a keyword statistics unit that finds statistics regarding an appearance of the keyword; a keyword assessment value calculation unit that calculates an assessment value of the extracted keyword based on the statistics regarding the appearance of the keyword; a determination unit that determines whether or not to register or delete the keyword based on the calculated assessment value; a dictionary registration and deletion unit which registers or deletes the keyword in or from a dictionary database based on a result of the determination performed by the determination unit; and the dictionary database.