Identification information of a speech spurt, hangover and pause is used to indicate that a digital voice signal is the speech spurt, hangover or pause. While the identification information of a speech spurt, hangover and pause is indicative of the speech spurt, a voice level adjuster does not attenuate the digital voice signal, and the voice signal/third signal combiner mixes it with a third signal which undergoes the maximum attenuation through a third signal level adjuster. While the identification information of a speech spurt, hangover and pause is indicative of the hangover, the voice level adjuster gradually attenuates the digital voice signal. This is because the level of the voice signal is expected to be high in the first half of the hangover period, but to decay in its latter half to such a level that it is dispensable for speech recognition. A third signal (noise), on the other hand, is gradually increased in the latter half of the hangover period to preserve the continuity in the transition from the speech spurt to a pause, thus achieving smooth transition to the pause. This makes it possible to reduce as much as possible the unnaturalness involved in switching between speech spurts and pauses, thereby improving the quality of the reproduced voice.