15. The speech dialogue system of claim 12, wherein the synthetic speech response cancellation means further comprises two channel A/D converter means for inputting the synthetic speech response generated by the synthetic speech response generation means and the microphone output signals outputted by the microphone means into the synthetic speech response cancellation means synchronously.
12. A speech dialogue system, comprising: microphone means for receiving a speech input uttered by a human speaker and outputting microphone output signals; speech recognition means for receiving input signals and recognizing the speech input received by the microphone means; synthetic speech response generation means for generating a synthetic speech response appropriate for the speech input recognized by the speech recognition means; loudspeaker means for outputting the synthetic speech response to the human speaker; and synthetic speech response cancellation means for cancelling the synthetic speech response, which is outputted from the loudspeaker means and then received by the microphone means, from the microphone output signals, to obtain input signals to be supplied to the speech recognition means from which the speech recognition means recognizes the speech input, the synthetic speech response cancellation means further comprising: adaptive filter means for adapting the synthetic speech response generated by the synthetic speech response generation means, to obtain an adaptive filter output; convolution means for calculating a convolution of the adaptive filter output obtained by the adaptive filter means and the synthetic speech response generated by the synthetic speech response generation means; subtractor means for subtracting the convolution calculated by the convolution means from the microphone output signals, to obtain the input signals supplied to the speech recognition means; at least one smoothing filter means for smoothing the synthetic speech response generated by the synthetic speech response generation means; and switching means for controlling an operation of the adaptive filter means according to an output power level of the smoothing filter means.