Provided is an electric power generating device capable of performing electret power generation by efficiently rotating a rotating electrode group. The electric power generating device includes: a plurality of electret electrodes each made of an electret material and formed into a planar shape, and arranged in a circular shape at intervals in a first plane; a plurality of counter electrodes arranged in a circular shape in a second plane facing the first plane so as to face the plurality of electret electrodes; an oscillating weight supported in a rotatable manner; and a motive power transmission mechanism configured to transmit motive power generated by rotations of the oscillating weight to a rotating electrode group including any one of the plurality of electret electrodes and the plurality of counter electrodes, to thereby rotate the rotating electrode group so that the rotating electrode group is moved relative to another electrode group, the motive power transmission mechanism including a clutch mechanism configured to transmit only a rotation of the oscillating weight in one predetermined direction.