a voltage drop as indicated by the broken line b in fig7 is present across the primary winding of the pulse transformer 40 and the following current flows therein equ2 thus in response to the sub station side transmission signal ss the current flowing in the primary winding of the pulse transformer 40 and accordingly the resistor 39 is changed as a result of which the potential at the point a changes in accordance with the current variation the potential at the point a is compared with the reference voltage vref by the comparator 38 and the output of the comparator 38 is employed as the master station side reception signal rm in this connection the reference voltage vref applied to the comparator 38 is set to a suitable value such as a level indicated by c in fig7 to detect the difference between the potentials a and b shown in fig7 as is apparent from the above description in the 34 reception mode 34 during the period of one time slot u03c4 the synchronizing signal is transmitted from the master station to the sub station and the data signal is accordingly transmitted from the sub station to the master station in accordance with the invention the variations of the current flowing in the primary side of the pulse transformer due to the variations in impedance of the secondary side of the pulse transformer are detected therefore it is unnecessary to provide pulse transformers for both the master station and the sub station that is the same effect for signal transmission can be obtained by providing a pulse transformer only in the master station as shown.