Disclosed is a generator wherein a rotary shaft is rotatably supported by a rotary shaft rest, and an inner casing is formed in a cylindrical hollow shape in such a way that one side thereof is fixedly engaged to the rotary shaft, thus rotating by the rotary shaft, and another side thereof is engaged to a rotary shaft rest by an inner casing engaging support member which includes a thrust bearing, and a plurality of through holes and ventilation holes are formed at the side surfaces and outer circumferential surfaces of the inner casing and the outer casing.