1. A word predicting method of predicting words in a predetermined sentence by using a computer, comprising the steps of: acquiring a history to be used in predicting words from a history storage means where a word prediction history is stored and specifying a sentence structure of said history; acquiring a context tree for word prediction from a context tree storage means, which stores said context tree having information about possible structures of a sentence and a probability of appearance of words with respect to said structures at nodes; and predicting a word to be predicted based on said specified sentence structure of said history and said acquired context tree for word prediction.