A tripping device of a circuit breaker of the present disclosure may reinforce a magnetic force on a main magnetic flux path using a plurality of magnetic force reinforcing plates without using an additional plate yoke on an actuator provided in the circuit breaker to miniaturize the size of the actuator, thereby having an effect of allowing a control circuit unit provided at an outer portion of the circuit breaker to be installed at an inner portion of the circuit breaker to miniaturize the whole size of the circuit breaker.