PURPOSE: To enable an indoor control circuit and an outdoor control circuit to interactively communicate with each other at high speed.

CONSTITUTION: When an outdoor control circuit 9 transmits signals to an indoor control circuit 8, an outdoor transmitting circuit 20 transmits '1' and '0' signals to an indoor receiving circuit 16 with voltages V1 and V2 of a direct current power source line S. When the indoor control circuit 8 transmits signals to the outdoor control circuit 9, an indoor transmitting circuit 18 transmits '1' and '0' signals to an outdoor receiving circuit 19 with electric currents I1 and I2 that flow along the direct current power source line S. During transmission, a load stabilizing circuit 17 prevents fluctuations in electric current level caused by operation state of equipment.