1. A speech recognition apparatus using a tree structured hypothesis having nodes each corresponding to a part of a recognition word, the apparatus adapted to calculate a likelihood of connected hypotheses expressed by a pass from a root node to a node of interest using acoustic and language scores, comprising: a language lookahead score generating unit configured to generate a language lookahead score for a node of interest based on the language score of each recognition word sharing the node of interest; an addition unit configured to add the acoustic score of the node of interest to the likelihood of connected hypotheses expressed by a path from the root node to a parent node of the node of interest; an updating unit configured to update the language lookahead score by deleting the language lookahead score when the parent node was the node of interest, and by adding the language lookahead score of the node of interest generated by said language lookahead score generating unit; and an updating interrupt unit configured to interrupt the updating of the language lookahead score by said updating unit, in accordance with the position of the node of interest in the tree structured hypothesis.