The circuit breaker is fabricated by an assembly step that makes a circuit breaker assembly with a moving contact metal plate, stationary contact metal plate, and bimetallic strip disposed in prescribed locations in an external case; and an anneal step that introduces the circuit breaker assembly made in the assembly step into an anneal oven, heats the circuit breaker assembly in the anneal oven and then cools it to anneal both the moving contact metal plate and the bimetallic strip and produce a heat-treated circuit breaker.