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. <EOS>