PURPOSE: To make it possible to operate a relay securely only during the time of passing within the section of a step-out locus, by detecting a step-out taking quantities of electricity of phases different from each other into step-out detecting elements, and obtaining a final output by the logical product of the outputs of both elements.

CONSTITUTION: A step-out detecting element 1 detects that a step-out locus 521 passes three regions Z1-Z3 having a specified stay, by taking in quantities VRS, IRS of electricity between phases. And, a step-out detecting element 2 detects that the step-out locus 521 passes three regions Z4-Z6 having a specified stay also, by taking in quantities VST, IST of electricity between phases. Then, both outputs of the step-out detecting elements 1, 2 are sent to a logical product element 3 in the next stage, and an output is generated. And, this prevents unnecessary operation of a relay caused by the loss, etc., of a quantity of electricity of an input, and makes it possible to operate the relay to a step-out within the section securely.