1. A speech recognition apparatus comprising: a storage medium for storing tree structured dictionary data that contains a plurality of words as nodes in a tree structure; a backward speech comparison unit for comparing a backward speech resulting from reproducing an input speech in chronologically backward order with a backward acoustic model corresponding to a speech of reversely reproducing a word string sequence toward a root of the tree structure, wherein a comparison is performed in reverse order of the sequence; a forward speech comparison unit for comparing the input speech with a forward acoustic model corresponding to a speech resulting from reproducing a leaf node word of the tree structure in chronologically forward order; and an output unit for outputting a word or a word string highly likely matching the input speech based on a comparison result each from the backward speech comparison unit and the forward speech comparison unit.