Disclosed is a circuit breaker monitoring a power line. The circuit breaker is provided with a power line monitoring circuit including a leakage current detecting circuit and a state judging circuit to monitor the on-off state of the shielding wire of the power line. When there is a leakage current between the phase and neutral wire of the power line and the shielding network wire is disconnected, the circuit breaker can automatically cut off the connection between the electrical appliance and the power supply to avoid the electric shock and fire accidents.