A similar document search method includes a step of extracting a characteristic word candidate as a candidate for a characteristic word from a seeds document including desired retrieval contents, a step of extracting as characteristic words of the seeds document, when the characteristic word candidate extracted by the extracting step is a compound characteristic word including a plurality of characteristic words, the compound characteristic word and constituent characteristic words included in the compound characteristic word from the characteristic word candidate, a step of calculating, according to the characteristic words extracted by the extracting step, similarity between the seeds document and a registration document, and a step of outputting as a retrieval result a result of the similarity calculated by the similarity calculating step.