PURPOSE: To obtain high recognition performance by detecting the level of the output of a gain controller and controlling the gain of the gain controller according to the level detection result.

CONSTITUTION: The gain controller 1 outputs a speech input signal without controlling the gain and a speech recognizing means 2 performs speech recognition without gain control, so the recognition rate is prevented from decreasing under the influence of variation in the level of the speech input signal. The gain controller 1 when receiving a gain control command signal controls the gain according to the level detection result from an input level detector 3 and performs proper gain control over a following speech input signal. Consequently, the gain is constant until the detection of a speech section is finished, so the features of the pattern of the speech waveform are never spoiled and the recognition rate increases. Further, the gain is controlled before a speech recognizing means 2 performs A/D conversion and the dynamic range of an A/D converter can be used effectively, so the recognition rate is improved.