PURPOSE: To detect detective contact occurring instantaneously on account of imperfect connection by connecting a voltage drop detection circuit to the rectified output side of a diode bridge, and inputting the output of this voltage drop detection circuit to a control circuit.

CONSTITUTION: A capacitor 9 is connected to the DC output side of the diode bridge 8, and when the connection part of the transmission line 5 is imperfect and turns into the detective contact instantaneously on account of vibration, etc., the inter input terminal voltage VA of the transmission line becomes 0V instantaneously, and the output voltage VB of the diode bridge 8 becomes 0V instantaneously as well, but the both terminal voltage VC of the capacitor 9 scarcely drops, and DC power to a DC power circuit 10 is supplied with no change. On the other hand, the output signal (a) of the voltage drop detection circuit 13 is inverted by the drop of voltage so as to inform a control part 11 of the occurrence of the detective contact. Thus, the instantaneous defective contact of the transmission line 5 can be detected, and the control part 11 can decide which of the defective contact or a noise the cause of the transmission error is.