1. A sentence segmentation method of using a computer to break text in a natural language into sentences, comprising the steps of: inputting the text to be processed, breaking the text into sentences through the use of dictionary data stored in a dictionary storage, and storing the broken text in memory; and reading said text to be processed from said memory, estimating the part of speech of a word constituting said text, and performing sentence segmentation processing on said text based on the result of the part-of-speech estimation.