In differential function calculation processing (1), a value of differential function f(d) is calculated using differences between a plurality of predetermined number p of differential current data Dd sampled at dif­ferent sampling times. The differential current data is data obtained from differential current Id or a sum of data Dj (j = 1 to n where n is the number of the respec­tive terminal currents) obtained from respective ter­minal currents Ipj. In restraint data calculation processing (2), restraint data Dr is calculated using respective terminal current data Dj sampled at the same sampling time. In restraint function calculation pro­cessing (3), a value of restraint function f(r) is calculated using differences between a plurality of pre­determined number p of restraint data Dr sampled at dif­ferent sampling times. Then, in comparison processing (4), the value of differential function f(d) is compared with the value of restraint function f(r). If the value of differential function f(d) is smaller than a value, modified by a predetermined relationship, of restraint function f(r), an inhibit output (S2 or f(s)) is gen­erated in the comparison processing (4). Differential relay processing (5) inhibits a trip operation of a cir­cuit breaker by the inhibit output (S2 or f(s)) from the comparison processing (4). The differential relay pro­cessing (5) is so controlled as to execute the trip operation when the inhibit output (S2 or f(s)) is not present.