A circuit breaker capable of opening and closing an electric path along which an AC current flows, the circuit breaker comprising: a current transformer configured to output a current signal in accordance with the AC current flowing along the electric path ; a reference waveform storage that has stored therein, in advance, reference waveforms of reference frequencies, the reference waveforms respectively corresponding to the frequencies, and one of the reference frequencies being equal to the frequency of the AC current; a multiplication unit configured to multiply a current waveform output from the current transformer with each of the reference waveforms stored in the reference waveform storage ; a Fourier transformer configured to perform Fourier transformation on each piece of multiplication data obtained by multiplication performed by the multiplication unit , and to thereby convert the multiplication data into frequency components; and a comparison determiner configured to determine whether or not a DC component is included in each piece of the multiplication data converted into frequency components by the Fourier transformer , and to detect a reference frequency, corresponding to multiplication data including the largest DC component, as the frequency of the AC current flowing along the electric path . a current transformer configured to output a current signal in accordance with the AC current flowing along the electric path ; a reference waveform storage that has stored therein, in advance, reference waveforms of reference frequencies, the reference waveforms respectively corresponding to the frequencies, and one of the reference frequencies being equal to the frequency of the AC current; the reference waveforms respectively corresponding to the frequencies, and one of the reference frequencies being equal to the frequency of the AC current; a multiplication unit configured to multiply a current waveform output from the current transformer with each of the reference waveforms stored in the reference waveform storage ; a Fourier transformer configured to perform Fourier transformation on each piece of multiplication data obtained by multiplication performed by the multiplication unit , and to thereby convert the multiplication data into frequency components; and to perform Fourier transformation on each piece of multiplication data obtained by multiplication performed by the multiplication unit , and to thereby convert the multiplication data into frequency components; and a comparison determiner configured to determine whether or not a DC component is included in each piece of the multiplication data converted into frequency components by the Fourier transformer , and to detect a reference frequency, corresponding to multiplication data including the largest DC component, as the frequency of the AC current flowing along the electric path . to determine whether or not a DC component is included in each piece of the multiplication data converted into frequency components by the Fourier transformer , and to detect a reference frequency, corresponding to multiplication data including the largest DC component, as the frequency of the AC current flowing along the electric path . <EOS>