1. A word predicting method of predicting words in a predetermined sentence by using a computer, comprising the steps of: retrieving a history tree of word prediction covering words before a word to be predicted to be used in predicting words from a word prediction history storage means where a word prediction history of partial parse trees is stored; dynamically selecting a shape of a partial parse tree as a word prediction reference range by specifying a sentence structure of said history; acquiring an arboreal 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; comparing at least one node of said arboreal context tree to said history tree; identifying a node of said arboreal context tree having a partial tree matched with said history tree; using a probability distribution appended to the identified node of the arboreal context tree to predict a word to be predicted; and outputting the predicted word.