An electric power steering system includes an electric motor and a worm speed reducer. The worm speed reducer includes a worm shaft, a worm wheel, and a ring member. The worm wheel includes an annular tooth portion and an annular support formed of a material that is softer than a material of a shaft. The ring member includes an outer periphery press-fitted with the annular support so as to form plastically deformed portions in part of the annular support, and an inner periphery with which the shaft is press-fitted. The ring member is formed of a material with a hardness equivalent to a hardness of the material of the shaft.