A rotating movement control mechanism including a worm, and a main driving side worm wheel and a driven side worm wheel disposed across the worm. Both end portions of the worm in an axial direction are axially supported by bearing parts. Respective teeth of the main driving side worm wheel and the driven side worm wheel contact both sides of the worm in a direction substantially orthogonal to the axial center with a backlash, and are supported thereby. When the second member is relatively rotated with respect to the first member, the worm rotates relatively with respect to the main driving side worm wheel. The driven side worm wheel rotates by a rotational force of the worm, and a rotational force of the driven side worm wheel is regenerated in the worm. Whirling vibrations of the worm are suppressed, and relative rotation is performed smoothly.