PURPOSE: To make a subscriber line maintenance free and to share a preliminary line by making the subscriber line into a light line concerning a subscriber line system to transmit the signal of a telephone, etc.

CONSTITUTION: A demultiplexing means 2 is provided at a multiplexer 1, a multiple signal and the signal of plural channels is multiplexed and separated, the signal of plural channels is connected to each subscriber terminal 3, and them, a light subscriber line interface means 4 is provided at the multiplexer 1, and the mutual conversion with the signal of plural channels and a small number of the channel multiplex light signals at a demultiplexer means 2 side is performed. A light subscriber line interface means 5 is provided at a relaying means 6 to connect the signal of plural channels to each subscriber terminal 3, the mutual conversion with the small number of the channel multiplex light signals and the signal of plural channels is performed, and the light subscriber line interface means 4 and the light subscriber line interface means 5 are connected by an optical transmission line 7.