In order to form a locking mechanism for a switch-on button of a circuit breaker which has a simple and inexpensive configuration, the locking mechanism has a locking element which is mounted rotatably on the switch-on button. The locking element, held directly in a first position by a first spring, enables actuation of the switch-on button and, directly in the switched-on state of the circuit breaker by a second spring, can be moved into a second position. The switch-on button of the circuit breaker is locked to prevent actuation in the second position.