PURPOSE: To secure an assured actuation of the selection circuit by adding the backup selection input circuit to the power source which is formed independently from the selection input power source to drive the change-over switch.

CONSTITUTION: Group selection bus 50 and individual selection bus 80 are provided between positive power source 10 and negative power source 20, and the switch and selection control is carried out by group selection bus change-over switch 104 and individual selection bus change-over switch 105. Then backup selection input circuits 101 and 102 are added to the power source formed independently from selection input power sources 10 and 20 which drive change-over switches 104 and 105, and thus OV is secured for the voltage applied to buses 50 and 80 at the nonselection time. As a result, the incapability of the selection control due to the loss of the power source and other factors can be solved, thus securing an assured operation for the selection circuit.