A leak current breaker is provided which includes a detector (10) to detect a leakage current from electric lines, a first removing unit (12) to convert the detected leakage current into a voltage and a voltage detector (14) to detect a voltage developed in electric lines A under testing, a second removing unit (16) to remove a harmonic component included in the voltage, a phase contrast detector (20) to detect a phase contrast from a signal waveform having the harmonic component removed, a frequency calculator (21) to calculate a frequency occurring on electric lines A on the basis of the signal waveform of the voltage having the harmonic component removed by the second removing unit (16), a phase angle calculator (22) to calculate a phase angle of the leakage current flowing through the electric lines A on the basis of the phase contrast and frequency, a root-mean-square value calculator (24) to calculate a root-mean-square value of the voltage having the harmonic component removed by the first removing unit (12), a calculator (27) to calculate a leakage current component Igr arising from an earth insulation resistance included in the leakage current flowing through the electric lines A on the basis of the root-mean-square value and phase angle of the leakage current, a judging unit (29) to judge whether the leakage current component Igr calculated by the calculator (27) has exceeded an arbitrary value, and a circuit breaker (30) to break the electric lines A on the basis of the judgment made by the judging unit (29).