PURPOSE: To provide a three-pole circuit breaker highly reliable for tripping operation and inexpensive, without any recourse to a special adjusting process, exclusive parts, or the like.

CONSTITUTION: Three single-pole type circuit breakers 1, each with a case 2 housing a bimetal and having an exhaust opening section 2A for discharging arc gas, are arranged in parallel. In this case, an interlock system is provided, so that when one of the breakers 1 trips, others also concurrently trip. Regarding these single-pole type circuit breakers 1, the opening area of the section 2A is formed in such a way that the center breaker has a larger area than others.