To detect a signal level of an image signal, etc. outside of a sitting room.

An apartment building interphone system comprises a plurality of sitting room masters 2, 2,... which are installed in the sitting room, and in which a resident takes an image of an image signal of a visitor in response to a call from a meeting porch apparatus for confirmation, to make a telephone call; and a controller 4 for controlling the meeting porch apparatus and the plurality of sitting room masters when a date signal, a telephone call signal and/or the image signal of the call and telephone call are modulated for transmission. The sitting room master comprises a level detection circuit 25 for detecting an image signal level, and a CPU 24 for transmitting the detected image signal level to a transmission path along with a sitting room number ID.