In a composite concentration system of the type utilizing a time division system in which voice signals are digitized and then concentrated in a traffic, there are provided a first subscriber line interface means of a first type which connects subscriber lines directly to subscriber line interface circuits, a second subscriber line interface means of a second type which connects subscriber lines to subscriber line interface circuits via a space division concentration switch and a time division concentration switch connected to the first and second subscriber line interface means via switching interfaces. The number of output terminals of the first subscriber line interface means is equal to that of the output terminals of the second subscriber line interface means when a concentration ratio of the space division concentration switch to the second subscriber line interface means is 1:1 and corresponding output terminals are constructed such that the same channel is assigned to the time division concentration switch. The composite concentration system of this invention is inexpensive and can readily expand a telephone system according to a variation in percentage of calls or change in a service performance.