To prevent malfunction in a control system for air conditioning apparatus in which an outdoor machine and an indoor machine are connected through a communication line by interrupting transmission and reception to a microcomputer of a connection instrument upon detecting communication failure by failure signal detection means when a transmission/reception processing is performed between the instruments on the communication line.

An outdoor machine A1 and an indoor machine B1 are connected onto a communication line D, and when power is supplied, the outdoor machine A1 transmits to the indoor machine B1 a signal for detecting communication failure by signal generator means a12 and an operation signal from an outdoor machine microcomputer a13 by outdoor machine transmission/reception means a11. In contrast, an operation signal is transmitted also to the outdoor machine A1 from the indoor machine B1 by indoor machine transmission/reception means b11 to operate a load a14 with the outdoor machine microcomputer a13. Hereupon, failure detection means b12 judges whether or not a waveform of a signal from the signal generator means a12 is normal, and issues an interruption instruction to communication interruption means b15 when a disturbance of the signal is detected to interrupt communication from the outdoor machine B1.