Abstract: Input signals supplied to a plurality redundant subsystems are input to an output selecting circuit after an output of a subsystem is changed from an output signal of a subsystem having a most reliable output signal to another output signal of another subsystem having a lower reliable output signal in turn using self-diagnoses and cross-diagnoses in each subsystem according to an algorithm as follows:if any subsystem has Syndrome 1 then select the subsystem(s) which has Syndrome 1,else if any subsystem has Syndrome 2 then select the subsystem(s) which has Syndrome 2;else if any subsystem has Syndrome L-1 then select the subsystem(s) which has Syndrome L-1;else output fail safe signal.The output selecting circuit outputs a selected signal by a method of majority voting based on the output signals from the subsystems.