PURPOSE: To enable control signals for an indoor unit and an outdoor unit to be transmitted to, and received by, the units, respectively, by a method wherein the switching element and the receiving element of the indoor unit and the switching element and the receiving element of the outdoor unit are connected to a single serial signal line and both of the switching elements are driven in accordance with a predetermined control signal pattern.

CONSTITUTION: The signal from an outdoor control circuit 20 received by the receiving element 17 is detected at both terminals of a resistor R6 and is inputted to an indoor control section 12. On the other hand, the signal from the indoor control circuit 10 detected by the receiving element 23 is detected at both terminals of a resistor R5 and is inputted to an outdoor control section 22. The transmission of the control signal for the outdoor unit of the air conditioner is made by driving the switching element 13 by the indoor control section 12 through a thyristor ignition circuit 14. On the contrary, in order to control the indoor unit, the switching element 25 is driven by the outdoor control section 22 through a thyristor ignition control 26.