A multiplex transmission system of this invention includes a plurality of multiplex nodes interconnected by a common multiplex transmission line. A plurality of groups of different combinations of the multiplex nodes are predetermined, and a frame of data is transmitted among the multiplex nodes through the multiplex transmission line. The data frame includes a group specifying area specifying a group of nodes from which an acknowledge signal is to be returned. When a data frame transmitted from a sending node is received, each multiplex node determines whether or not it belongs to the group specified by bits in the group specifying area of the transmitted frame. When the data frame is properly received, only those multiplex nodes which belong to the specified group return the acknowledge signals to an acknowledge signal area in the data frame in accordance with the bits in the group specifying area, whereby the transmission efficiency is improved.