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.