To provide an electric power steering device improving the strength of a synthetic resin worm wheel transmitting the rotation of an electric actuator for generating a steering auxiliary force, enabling the high output of the electric actuator, reducing the diameter of the worm wheel, and saving a space.

The rotation of an electric actuator 8 for generating a steering auxiliary force is transmitted to wheels 6 via a worm 9 and a worm wheel 10. The worm wheel 10 is made of a synthetic resin material. The number average molecular weight of the synthetic resin material is set to not less than 30,000 and not more than 60,000.