To provide a circuit breaker for a single-phase three-wires with neutral conductor phase-interruption protection, capable of preventing omission of the connection of a lead wire for phase interruption detection to the neutral wire.

A voltage-dividing circuit 3 which outputs voltages obtained by dividing voltage from a main circuit to a failed-voltage detection circuit 2 is formed by connecting a third resistor R3 between one L2 of a voltage line and a lead wire 6 for phase interruption detection, in addition to a first resistor R1 and a second resistor R2 for dividing the voltage of the main circuit. The resistance value of the third resistor R3 is selected so that the voltage divider voltage with a total sum of the third resistor R3 and the first resistor R1 under a condition where the lead wire 6 for phase interruption detection connected with one end of the first resistor R1 is not connected with a neutral wire N and by the second resistor R2 may exceed a trip voltage determined as failure.