To provide a circuit breaker for a low-tension power contract capable of activating an overcurrent detection circuit at a driving voltage obtained from a current transformer.

The circuit breaker for low-tension power contract is provided with a current transformer 15 detecting a load current flowing in a main-circuit wire 131, an overcurrent detection circuit 17 detecting overcurrent based on an output of the current transformer 15, and a driving voltage circuit 19 supplying driving voltage to the overcurrent detection circuit 17. The current transformer 15 is made provided with a first coil 151 for detecting a load current, a second coil 152 for generating driving voltage, and an annular core 153 magnetically coupling the coils 151, 152. A terminal of the first coil 151 is connected to the overcurrent detection circuit 17, and that of the second coil 152 is connected to the driving voltage circuit 19. It is to be so structured that a current transformer 15 is provided for each of the plurality of wires and the driving voltage circuit 19 can obtain driving voltages from the plurality of current transformers 15.