PURPOSE: To smooth the work by eliminating redundant information from a voice signal with a compressor and sending the resulting signal to a communica tion line.

CONSTITUTION: A voice signal is converted into a digital signal by an input means 10. A microphone 12 transduces a sound pressure signal into an electric signal, after the signal is amplified and given to a low pass filter 16, in which excess high frequencies in excess of the sampling frequency are eliminated and an A/D converter 18 is used to convert the signal into a digital signal by a proper sampling time. A voice conversion circuit 20 applies code compression to the digital signal and stores the signal and employs a PCM circuit 22 to apply pulse code modulation and to eliminate redundant information and the signal is compressed into a data quantity able to be sent through a communication line 40. Thus, voice information is sent in real time by using a communication line for air-conditioning control and it is not required to provide an independent interphone line and a smooth work is attained.