Circuit breaker

A circuit breaker capable of breaking an associated circuit when an overcurrent higher than a rated current keeps flowing through the breaker or upon a short-circuit current. Electric power path in the breaker is partly inserted through an arm of a magnetic material and holding a movable contactor plate of make-and-break contact means, an armature means is provided as pivoted at one end to the holding arm, and a tension spring means is provided between the other end of the armature means and the movable contactor plate to provide a contact pressure. The overcurrent flowing through the path causes a magnetic force to be induced in the holding arm, whereby the armature means is rotated to change the direction in which the force of the tension spring means acts on the movable contactor plate and the latter breaks the contact means.