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

This electromagnetic tripping device is structured so that a shaft 19a is provided in each support part 11 of a support member 10, and a groove 29 to make insertion of the shaft 19a is formed in that part of each bearing parts 24a, 24b of a movable core 20 which is located on the some side as that of the pulling direction of a return spring 1, and thereby the core 20 can easily be journaled to the support member 10.