To provide a distance relay system where impedance to an accident point can be calculated with sufficient precision even if a frequency of a power system fluctuates and impedance can be calculated irrespective of a sampling phase.

Instantaneous values of voltage and current in a power system are stored in a memory part 1 and an extracting part 2 extracts data on a prescribed period. A plurality of operation parts 3, 4 and 5 perform a prescribed operation on instantaneous value data. A judging part 6 outputs a protection operation signal when a final operation result is within a range of a prescribed threshold. Instantaneous value data on reference time, instantaneous value data at a point before 90 in an electrical angle of a rated frequency and instantaneous value data at a point before 180 in the electric angle of the rated frequency are extracted. A plurality of operation parts calculate a difference of two different effective value operation results from instantaneous value data and correct frequency fluctuation of the power system.