PURPOSE: To prevent the confusion in a fault detection and an operation, etc.

CONSTITUTION: For packages Q1 to Q3, signals K1 to K3 are connected with the input terminals on one side of selectors 2-1 to 2-3. The input terminals on the other side of the selectors 2 l 1 to 2-3 are connected with the dummy signals from a dummy signal generator 1. A reserve package Q4 is connected with signals K1 to K3 via a selctor 2-4 and further is connected with the dummy signal via the selector 2-4. The dummy signal has the monitoring pattern of the same phase as the signals K1 to K3 and the folding signal fixed to 'non- operation' is superposed on the dummy signal. By operating the selectors 2 and 3, the dummy signal is inputted in the reserve package Q4 or the package in switch of the packages Q1 to Q3.