PURPOSE: To attain the mode selection from a primary station by transmitting a synchronizing pulse for time division transmitted from the primary station while switching the voltage level in response to the mode to be selected.

CONSTITUTION: The synchronizing pulse transmitted from the primary station is switched into two stages of levels in response to the model to be selected by a counter 12 and also the synchronizing pulse is counted by the secondary station. When it is detected that the own address is coincident with the counter, a voltage level detection circuit 13 discriminates the voltage level of the synchronizing pulse and switches selectively the operating mode of the secondary station depending on the voltage level. Since no special control for mode selection is required in this way, no special transmission line is added even to a system adopting multi-drop connection by means of two-cored private line between the primary station and plural secondary stations, allowing to add the mode selection function.