A subject of the present invention is to provide an electric power steering capable of improving a contact ratio by using an hourglass worm to attain a higher output, and also performing a correction of a misalignment easily by simplifying remarkably a fitting operation of the hourglass worm. A worm gear system causes a hourglass worm 2 driven by an electric motor 4 to mesh with a worm wheel 3 provided to an output shaft 5, and a bearing 7 on the shaft end side, which bears rotatably the hourglass worm 2 is composed of a tapered roller bearing an outer ring of which can be separated. In a fitting operation, an inner ring 7a and rolling elements 7b are fitted into the hourglass worm 2 and also an outer ring 7c is fitted in advance in a gear housing 1. Then, the tapered roller bearing 7 is assembled in the gear housing 1 by moving the hourglass worm 2 obliquely to a rotating shaft line of the hourglass worm 2 in the gear housing 1 along a raceway of the outer ring 7c.