Multiplex transmission system

A multiplex transmission system of this invention includes a plurality of multiplex nodes(11-15) interconnected by a common multiplex transmission line(16). A frame of data is transmitted among the multiplex nodes(11-15) through the multiplex transmission line(16). When a data frame transmitted from an air conditioner switch multiplex node (subnode)(13) is received, an air conditioner unit multiplex node (main node)(12), which is predetermined as a destination node, detects the contents of each data from an ID-a of the received data, performs a calculation on SOM to DATA4 received, generates collation data (CRC) in accordance with the result of the calculation and transmits same. The subnode(13) generates collation data (CRC), which is an error check code, through a calculation on the transmitted SOM to DATA4, and compares the thus-generated CRC with the CRC received from the main node, to determine whether or not the data transmission was properly carried out. When the data transmission was carried out properly, the subnode(13) returns a signal ACK3, and when the data transmission failed, the data frame is retransmitted, whereby the subnode(13) need not have an ACK management function.