A theme phrase feature value calculation unit 103 calculates a feature value Fq of a given theme phrase Q based on the cosine similarity with a feature keyword of a feature keyword DB 107b. A positive/negative example determination unit 104 applies the feature value Fq to a positive/negative example determination model to determine whether a response phrase A is a positive example or a negative example. A response phrase score calculation unit 105 calculates the cosine similarity between the theme phrase Q and a plurality of theme phrases that form a pair with a response phrase A determined to be a positive example, and the highest value of the calculated cosine similarity is set as a score of the response phrase A. A response phrase selection unit 106 selects the response phrase indicating the maximum score as the response phrase for the theme phrase Q.