1 ) A word boundary probability estimating device comprising: means for calculating a probability that a word boundary will exist between a first plurality of characters constituting a first character string stored in a first corpus by invoking information as to whether a word boundary already exists between the first plurality of characters from the first corpus containing the first character string comprising the first plurality of characters or setting up the preliminary information as to whether the word boundary exists, and means for estimating the probability that the word boundary will exist in a second plurality of characters constituting a second character string stored in a second corpus by referring to a calculated probability between the second plurality of characters constituting the second character string stored in the second corpus.