1. A method for use with a speech recognition device for improving speech recognition performance, said method comprising: identifying a start position of a speech region of speech data for which speech recognition is to be performed; generating, from said speech data for which speech recognition is to be performed, a plurality of pieces of speech data including said speech region and a varying period of a preceding non-speech region, where start positions of non-speech regions differ for the plurality of pieces of speech data; performing speech recognition using each of said pieces of speech data to obtain a plurality of recognized results; and identifying a most numerous recognized result from among the plurality of obtained recognized results; wherein, by sequentially shifting the start position of said non-speech region from the start position of the speech region back to a position preceding by a predetermined time, a plurality of pieces of speech data whose start positions of non-speech regions differ are generated from said speech data for which speech recognition is to be performed.