To provide an electromagnetic tripping device of a circuit breaker of a structure for which a movable iron core can easily be pivoted on a support member.

An electromagnetic tripping device is structured that a shaft 21a is provided protrusively from each of a pair of support parts 24a and 24b formed on both sides of the central part substantially of a movable iron core 20, and a groove 12 to cause insertion of the shaft 21a is formed in that part of each of a pair of supporting parts 11 of a support member 10, which is located on the side opposite the pulling direction of a return spring 1, and thereby the core 20 can be journaled easily to the support member 10.