To provide a worm reduction gear having ideal steering performance with less steering noises by reducing gearing noises in a gear non-engaging area.

In a grinding range (D-range) of a worm 1, the three-stitch dimension of the worm 1 is partially changed. Specifically, the three-stitch dimension of the worm 1 is set as a value to be compete with a worm wheel 2 in a preload engaging condition in a gear engaging area (A-range) to suppress backlash while it is set as a value to be distant from the worm wheel 2 in a non-engaging area (B1-or B2-range). Thus, at a gear approaching portion (C-portion) in the non-engaging area, even when the worm 1 is displaced in a thrusting direction (arrowmark) by the assist of an electric motor 3, gearing noises and prevented and steering noises are reduced.

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