To provide a circuit breaker including a cord short circuit detection circuit for detecting only cord short circuit.

The circuit breaker includes: sensor wiring 9 detecting one circuit voltage of an outlet 4; a peak value detection circuit 20 detecting a peak value of a voltage drop amount from a detected voltage of the sensor wiring 9; an average value circuit 21 averaging detected peak values; and a determination circuit 22 determining that the cord short circuit occurs when the averaged peak value falls below a predetermined value. When the determination circuit 22 outputs a cord short circuit detection signal, trip operation of a trip coil 12 is carried out to interrupt a circuit, and a display part 16 indicates that interruption is caused by the cord short circuit.