Disclosed embodiments include an instant trip mechanism for a molded case circuit breaker. In some embodiments, the mechanism includes an adjustment dial to set a current for an instant trip operation; an instant bar provided with an upper portion contactable with the adjustment dial, a shaft portion serving as a rotation shaft, and a lower extending portion downwardly extending from the shaft portion; an electromagnet unit to generate a magnetic attraction force that is proportional to an amount of current flowing on the circuit; an armature rotatable with a lower end portion supported by a shaft, and attracted toward the electromagnet unit by the magnetic attraction force; and a spring for applying to the armature a load varying in a direction of the armature getting away from the electromagnet unit.