PURPOSE: To prevent malfunction due to line quality deterioration in its own line by providing a mark rate modulation instruction demodulation circuit and a parity violation instruction selector switch controlled by a demodulated instruction.

CONSTITUTION: A parity violation(PV) input selection instruction by mark rate modulation (MDM) is inputted to an MDM instruction demodulation circuit 2 by an MDM instruction input 9 and the instruction is demodulated. For example, when an instruction to select its own line input 6 is demodulated, a switch control circuit 3 controls a PV instruction selector switch 4, which is thrown to the position of the PV instruction own line input 6. Then the PV instruction from its own line is inputted to a PV instruction demodulation circuit 5 and the demodulated instruction is outputted to an instruction output 10.