1. A pronunciation variation rule extraction apparatus comprising: a speech data storage unit which stores speech data; a base form pronunciation storage unit which stores base form pronunciation data representing base form pronunciation of said speech data; a sub word language model generation unit which generates a sub word language model from said base form pronunciation data; a speech recognition unit which recognizes said speech data by using said sub word language model; a difference extraction unit which extracts a difference between a recognition result outputted from said speech recognition unit and said base form pronunciation data by comparing said recognition result and said base form pronunciation data; and a language model weight control unit which controls one weight value for said sub word language model, wherein said language model weight control unit outputs a plurality of said weight values, wherein said speech recognition unit recognizes said speech data for each of said plurality of weight values, and wherein said language model weight control unit determines whether or not said weight value should be updated, based on said difference when said difference is extracted.