a wind driven electrical power generating apparatus includes a rotor rotating by receiving wind force , a gear assembly having an input shaft connected to the rotor , a generator connected to an output shaft of the gear assembly , a sensor for detecting generating capacity of the generator and a controller for varying gear ratio of the gear assembly based on a signal from the sensor . in this construction , the controller controls so that the generator keeps rotating at around the lowest rotational speed within a range of rotational speed , the range being determined so as to include a maximum power operation region of the generator . alternatively , the controller controls so that rotational speed of the output shaft of the gear assembly is adjusted to at around the lowest rotational speed within a range of rotational speed , the range being determined so as to include a rated power region as an upper operational limit .