PURPOSE: To multiplex only necessary data without multiplexing unnecessary data received from a communication line which is in a communication stoppage state.

CONSTITUTION: A multiplex circuit is provided with a multiplex control circuit 4 inputting communication request signals f, g showing that communication lines a, b are in a communication or in a communication stoppage state, outputting only line data c which is necessary data from a multiplex circuit 3 without multiplexing line data d from a communication line b in a communication stoppage state, for instance, the communication line b, and performing a control that the clock rate of the communication line a is speeded up for allotting the data for the portion which the line data d is not multiplexed to the line data c.