To perform an alternative operation in a manner of survival against emergency state in an air conditioner constituted by a plurality of outdoor devices and a plurality of indoor devices which are connected on the same piping and the same communication line by a method wherein when one outdoor device communicating with a certain indoor device stops its transmission, the other outdoor device changes its address.

An outdoor device A2 is provided with a transmission/receiving means a21, an address setting means a23, an address changing means a24, a stopping timer a25 and a micro-computer a26. The address changing means a24 changes a setting attained at the address setting means a23 and changes the address of the outdoor device A2 from 'No.A2 machine' to 'No.A1 machine', thereby performs a communication with the indoor devices B1, B2 in the same manner as that of the outdoor device A1 and performs an operation like a survival action against emergency case against the indoor devices B1, B2.