A mold cased circuit breaker is disclosed that enables a user to easily discern an actually connected state between stationary contactors and movable contactors and forcibly separates a connection therebetween if there arises electrical fault, wherein the circuit breaker comprises: a first isolation link hinged at a lower end thereof to a shaft unit and selectively engaged at an upper end thereof with a lateral surface of a lever; a second isolation link centrally formed with an oblong hole for accommodating a guide pin protruded on a side plate and linked at one end thereof to an upper end of the first isolation link; and an isolation lever adjacently positioned at one end thereof to the other end of the second isolation link, and so positioned at the other end thereof as to selectively engage with a nail, and rotatably mounted at the side plate to rotate the nail by way of operation of the second isolation link.