PURPOSE: To make a complicated manual operation for the connection switching of a monitor equipment unnecessary by attaching a voice monitoring part consisting of a channel selection means, a memory, a D/A converter, and a speaker.

CONSTITUTION: A channel selection circuit 71 selects a channel designated on two, up and down, highways by channel selective designation from a maintenance engineer, and outputs the voice data of a selected channel. The memory 72 writes up/down voice data outputted from the selection circuit 71 sequentially, and simultaneously, reads out written voice data sequentially. Read out voice data is converted to an analog signal by the D/A converter 73, and simultaneously, both voice signals of up and down highways are coupled. A coupled signal is inputted to the speaker 74. Then, it is vocalized from the speaker. In such a way, it is possible to monitor talking voice.