PURPOSE: To facilitate a diagnosis and to prevent a break of a switch during the diagnosis, by performing the diagnosis is two modes.

CONSTITUTION: A diagnosing circuit 5 is coupled with a data memory 6 and a read/write circuit 4 in a PCM signal connection swtich 1. In the 1st mode, a connection state in a diagnosis is read, written, and read again to be compared with the 1st read state, thus deciding on the propriety of a holding memory 2 for a switch group 3. In the 2nd mode for a high-precision diagnosis, the current connection state of the switch group 3 is read out of a memory 2 and bits "1" and "0" of the read data are inverted, written, and read again to be compared with the written state, deciding on the propriety of the memory 2. After the diagnosis, the 1st read data is written, and the state of the memory 2 is returned to before the diagnosis.