The present invention consists in a circuit breaker wherein a pair of rigid conductors which are disposed therein and at least one of which has a movable portion that separates a pair of contacts on the basis of the electromagnetic force of an excess current flowing through the contacts adapted to come into and out of contact with and from each other, are provided with arc shields in a manner to surround the contacts, thereby to bring forth the effects of increasing the arc voltage of an electric arc struck across the contacts and raising the separating speed of the contacts.