A voice separation means 82 separates an input voice of a volume adjusted by an input volume adjustment means 81, into a voice recognition voice and a monitoring voice. A monitoring volume adjustment means 83 adjusts a volume of the monitoring voice. An output volume adjustment means 84 adjusts a volume of an output voice and causes an output device to output the output voice of the adjusted volume, the output voice being a voice obtained by synthesizing a synthetic voice and the monitoring voice of the volume adjusted by the monitoring volume adjustment means 83, the synthetic voice being a voice synthesized from information generated as a result of voice recognition of the voice recognition voice. A control means 85 instructs the monitoring volume adjustment means 83 to adjust the volume of the monitoring voice so that an amplification factor of the volume of the output voice with respect to the volume of the input voice does not exceed 1.