The invention relates to a safety lock (1) for an elevator landing door detecting intrusion of a person in the shaft through the landing door, activating a bistable switch (21) through its spindle lever (11) putting the elevator into safe operation when the landing door is opened by turning the corresponding key, it being impossible to reset the switch (21) manually from the inside of the shaft, which comprises a device (17) fixing the lock in the open position with the door open, that is released when the door is closed such that the switch (21) can be reset by turning the lock (1) in the closing direction of the door in which the key is turned in the opposite direction to the opening direction, the switch (21) being reset mechanically by a mechanical means connected to the spindle lever (11).