PURPOSE: To attain variable channel constitution for each call in response to the request by a subscriber by using a fixed multiplex circuit, a distribution switch network, a variable multiplex circuit and a fixed multiplexer/demultiplexer circuit in combinations.

CONSTITUTION: The transmission capacity of a subscriber transmission line is selected to be an integral number of multiple of a basic transmission capacity R1. An information signal from a terminal equipment 1 whose transmission capacity exceeds the value R1 is demultiplexed by the fixed multiplexer/demultiplexer in the unit of R1 and information signals from plural terminal equipments are distributed to plural variable multiplexer circuits 16 by a distribution switch network 15. The distribution switch network selects the variable multiplexer circuit 16 corresponding to the subscriber request channel constitution varying with each call and the variable multiplexer circuit 16 arranges the information signals from the plural terminal equipments to be distributed to a multiplex frame of the capacity R1. The multiplexed frame subject to arrangement is further multiplexed by the fixed multiplexer circuit 17 to constitute the transmission line multiplex frame. Thus, the change in the channel constitution of the multiplex frame is realized easily in response to the request from the subscriber.