A protective relay system for protecting a power system includes a selection circuit, a comparing circuit, and a data processing unit. The selection circuit time-sequentially switches a plurality of input signals having electrical quantities from the power system under control of the data processing unit and applies the input signals to the comparing circuit. The comparing circuit applies to the data processing unit a discriminated output signal representing the result of the comparing of a level of a signal having an electrical quantity and obtained from the input signals to the comparing circuit with a reference level. At a time point upon lapse of a time corresponding to an electrical angle of 90.degree. of the input signal, for example, after the logical level of a first discriminated output signal corresponding to a first controlled state of the selection circuit has been changed, the comparing circuit produces a second discriminated output signal corresponding to a second controlled state of the selection circuit different from the first controlled state. The data processing unit determines the level of a protection output signal according to a logical level of the second discriminated output signal.