PURPOSE: To attain broadcast among rooms with simple constitution by connecting a microphone of each interior to a speaker via a mixer and connecting a mixer in an adjacent room via a switch.

CONSTITUTION: When rooms A, B, C and D are used individually, switches 1 for next room interconnection are all opened, and a voice inputted from a microphone 3 is outputted from a speaker 6 via a preamplifier 4, a mixing circuit network resistor group 2 of a mixer 2 and an amplifier 5. When the rooms A, B are used at the same time by removing a partition E, the voice from the microphone 3 of the room A is transmitted to the next room B by a two-core shield cable for interconnecting each room in turning on the switch 1 for next room interconnection of the room A. Resistors R1, R2 of the room A are overall impedance matching devices for the room A and the next room B to keep the fluctuation of the voice level in a constant value through analog addition and subtraction.