A residual-magnetic-flux calculating unit includes a voltage-change-rate detecting unit that detects a transformer-voltage change rate from a phase voltage between a breaker and a transformer, a residual-magnetic-flux detecting unit that detects a residual magnetic flux remaining on the transformer based on the phase voltage, and a breaker-switching-state identifying unit that detects a switching state of the breaker. The residual-magnetic-flux calculating unit recalculates the residual magnetic flux based on the transformer-voltage change rate and a predetermined threshold while the breaker is in an open state.