PURPOSE: To conveniently use a multiplex transmission line by automatically constituting an optimum multiple constitution in accordance with the gathered communication volume with respect to the kind of trunk circuit, to which importance is attached, in the multiplex transmission line.

CONSTITUTION: A communication volume gathering means 4 is used to gather the momentary communication volume with respect to a specific kind of trunk circuit to which importance is especially attached out of plural kinds of trunk circuits for data communication, audio communication, picture communication, etc., multiplexed on a multiplex transmission circuit 10. The communication volume of each kind of trunk circuit and a multiple constitution correspondence table 1 are referred in accordance with the gathered communication volume to control use permission/inhibition of individual trunk circuits by a control means 2. Thereafter, the multiple constitution of plural kinds of trunk circuit is changed by a multiple constitution control part 3 incorporated in the multiplex transmission equipment 10. Thus, the communication volume of a specific kind of trunk circuit to which importance is attached is automatically secured.