To provide a terminal part structure of a circuit breaker, which can improve accuracy of an overcurrent tripping time with a simple structure.

The terminal part structure of the circuit breaker includes a terminal seat arranged on an inner side of an approximately box-shaped terminal metal fitting, wherein the terminal seat includes an approximately circular part formed on a terminal port side, and a bimetal mounting part formed on a back side of the approximately circular part. A terminal screw is inserted from an upper surface of the terminal metal fitting, and the terminal screw is fastened to connect a conductor to the terminal seat. A bimetal for operating a handle unit when the overcurrent flows through the circuit is connected to the bimetal mounting part provided on the terminal seat.