A rotation movement control mechanism comprising: a worm (50; 50A) pivotally supported between a pair of bearing parts (510, 520; 510A, 520A) arranged in both end portions in an axial direction by any of a first member and a second member, so that the second member is controlled rotatably with respect to the first member and maintained at an appropriate rotation angle by the rotation movement control mechanism; and a driving worm wheel (40; 40A) provided in any one of the first member and the second member and a driven worm wheel (60; 60A) provided in the other of the first member and the second member, so as to engage with the worm in a positional relationship across the worm, when the first member and the second member are rotated relatively, the worm engaged with the driving worm wheel rotates and the driven worm wheel engaged with the worm also rotates, at the time of locking, the driven worm wheel is moved in a direction of being pushed against the worm, and thereby locking can be performed in a state where backlash between both is reduced, the mechanism has rotating fulcrums (50a, 50b; 50A1, 50A2) that are provided in both end portions in the axial direction and are fulcrums in rotation of the worm, and a locking fulcrum provided in a different position from the rotating fulcrums in one end portion in the axial direction, at the time of rotation, each of the rotating fulcrums of both end portions in the axial direction is supported by the pair of bearing parts, at the time of locking, by the worm being displaced in the axial direction according to the movement of the driven worm wheel in the direction of being pushed against the worm, the locking fulcrum is supported in one end portion in the axial direction and the rotating fulcrums are supported in the other end portion in the axial direction, by the pair of bearing parts, and the mechanism has a structure in which an angle of lead of the worm at the time of locking is changed to be smaller than an angle of lead at the time of rotation. a worm (50; 50A) pivotally supported between a pair of bearing parts (510, 520; 510A, 520A) arranged in both end portions in an axial direction by any of a first member and a second member, so that the second member is controlled rotatably with respect to the first member and maintained at an appropriate rotation angle by the rotation movement control mechanism; and a driving worm wheel (40; 40A) provided in any one of the first member and the second member and a driven worm wheel (60; 60A) provided in the other of the first member and the second member, so as to engage with the worm in a positional relationship across the worm, when the first member and the second member are rotated relatively, the worm engaged with the driving worm wheel rotates and the driven worm wheel engaged with the worm also rotates, at the time of locking, the driven worm wheel is moved in a direction of being pushed against the worm, and thereby locking can be performed in a state where backlash between both is reduced, the mechanism has rotating fulcrums (50a, 50b; 50A1, 50A2) that are provided in both end portions in the axial direction and are fulcrums in rotation of the worm, and a locking fulcrum provided in a different position from the rotating fulcrums in one end portion in the axial direction, at the time of rotation, each of the rotating fulcrums of both end portions in the axial direction is supported by the pair of bearing parts, at the time of locking, by the worm being displaced in the axial direction according to the movement of the driven worm wheel in the direction of being pushed against the worm, the locking fulcrum is supported in one end portion in the axial direction and the rotating fulcrums are supported in the other end portion in the axial direction, by the pair of bearing parts, and the mechanism has a structure in which an angle of lead of the worm at the time of locking is changed to be smaller than an angle of lead at the time of rotation.