1. A word boundary probability estimating device comprising: a memory and a processor device; a first corpus comprising a first character string comprising a plurality of characters, the characters word-segmented manually; a second corpus, the second corpus larger than the first corpus and comprising a second character string, the second character string comprising a plurality of word-unsegmented characters; means for calculating a probability that a word boundary exists between each of the plurality of characters in the first character string, the calculating comprising: determining if a sequence of characters matching the plurality of characters already exists in the first corpus; determining if a word boundary already exists between the plurality of characters in the first character string in response to determining that a matching sequence exists; calculating the probability that a word boundary exists in the plurality of characters based on relations in the sequence of character types of successive characters in the first character string in the first corpus by using only the probability of a word boundary already existing between characters of those character types in response to determining that a word boundary already exists between the plurality of characters in the first character string; and using a probability of 0.5 in response to determining that the sequence matching the plurality of characters does not exist in the first corpus; and means for estimating the probability that a word boundary exists in the plurality of non-word-segmented characters in the second corpus by referring to the calculated probability of the boundaries between each of the characters in the characters of the first character string.