To provide an electronic overcurrent circuit breaker doing away with complicated wirings, which can mechanically activate a built-in electromagnetic breaker to break a current path even in case an electronic monitor fails to work at short circuiting, and which can freely carry out change of setting of a rated operating current or an instantaneous operating current in response to a load.

Current detectors 14, 15 provided on a current path 11 between the electromagnetic breaker 13 and the load 21, the electronic monitor 16 for tripping the electromagnetic breaker 13 based on detection signals from the current detectors 14, 15, and a power source circuit 17 supplying a power source to the electronic monitor 16 are to be provided, of which, the electromagnetic breaker 13 is to be provided with both a signal-type breaking mechanism 18a carrying out current-path breaking based on breaking signals of the electronic monitor 16, and a mechanical breaking mechanism 18b mechanically breaking the current path at short circuiting independently of the electronic monitor 16.