A worm biasing structure that biases a worm toward a worm wheel in a biasing direction, includes: a radial bearing that supports the worm in a radial direction; a bearing holder that has a holder hole for accommodating the radial bearing; a biasing member for biasing the radial bearing toward the worm wheel; and a housing that has a housing hole for accommodating the bearing holder, a pair of guide surfaces is formed as defined herein, when the bearing holder is accommodated in the housing hole and the radial bearing is not accommodated in the bearing holder, a distance between the pair of guide surfaces is smaller than or equal to an outer diameter of the radial bearing, and a gap is formed between an outer surface on the outer side of the guide surface and the housing hole.