PURPOSE: To enable simplification of a switching portion to be promoted and at the same time prevent damage of an equipment due to erroneous switching.

CONSTITUTION: Separation between a transmission-side constant-current circuit 1 which is connected from a private line L1 and a data from a transmission control portion 2, separation between a reception-side constant-current circuit 4 and a reception control portion 5 which are connected from a private line L2 and a line ground, and connection between the reception-side constant-current circuit 4 and the reception control portion 5 which are connected from the private line L2 and the transmission-side constant-current circuit 1 connected from the private line L1 are performed simultaneously.