PURPOSE: To improve line quality by inserting an address to an input signal at the input part of a cross connect part, detecting the address at an output part and comparing the inserted address with the detected address so as to detect line error.

CONSTITUTION: For a cross connect circuit 3, when the signal inserted addresses outputted from respective address inserting circuits 9a-9n is inputted, a path is set between the input and the output, and the signal passed through this path is outputted from the cross connect circuit 3 and inputted to address detection circuits 10a-10n. The address detection circuits 10a-10n detect the address inserted to the inputted signal and output it to a switching control circuit 6. The switching control circuit 6 compares the address inserted on the input side of the cross connect circuit 3 with the address detected on the output side and detects the line error. Thus, line quality can be improved.