A digital protection relay to which AC current flowing through a transformer is inputted and which performs an interruption operation of a circuit breaker connected to the transformer when overcurrent is detected, the digital protection relay comprising: an A/D conversion unit configured to sample the inputted AC current at regular intervals; a calculation processing unit configured to perform frequency analysis by Fourier transform on the basis of a digital value sampled by the A/D conversion unit; and a control unit configured to inhibit the interruption operation of the circuit breaker when a proportion of a harmonic extracted by the calculation processing unit is equal to or greater than a threshold set on the basis of a proportion of a harmonic contained in excitation inrush current, characterized by: a threshold learning unit configured to, perform a learning operation for updating the threshold, wherein after a signal for starting the learning operation is input to the threshold learning unit, the control unit is commanded to turn the circuit breaker on, of at least three waves of excitation inrush current occurring when the transformer is connected, a wave having a predetermined current value or higher is subjected to the frequency analysis by the calculation processing unit to calculate a harmonic content rate for each wave, the threshold is updated on the basis of the harmonic content rates obtained by the frequency analysis. an A/D conversion unit configured to sample the inputted AC current at regular intervals; a calculation processing unit configured to perform frequency analysis by Fourier transform on the basis of a digital value sampled by the A/D conversion unit; and a control unit configured to inhibit the interruption operation of the circuit breaker when a proportion of a harmonic extracted by the calculation processing unit is equal to or greater than a threshold set on the basis of a proportion of a harmonic contained in excitation inrush current, characterized by: a threshold learning unit configured to, perform a learning operation for updating the threshold, wherein after a signal for starting the learning operation is input to the threshold learning unit, the control unit is commanded to turn the circuit breaker on, of at least three waves of excitation inrush current occurring when the transformer is connected, a wave having a predetermined current value or higher is subjected to the frequency analysis by the calculation processing unit to calculate a harmonic content rate for each wave, the threshold is updated on the basis of the harmonic content rates obtained by the frequency analysis. a threshold learning unit configured to, perform a learning operation for updating the threshold, wherein after a signal for starting the learning operation is input to the threshold learning unit, the control unit is commanded to turn the circuit breaker on, of at least three waves of excitation inrush current occurring when the transformer is connected, a wave having a predetermined current value or higher is subjected to the frequency analysis by the calculation processing unit to calculate a harmonic content rate for each wave, the threshold is updated on the basis of the harmonic content rates obtained by the frequency analysis.