A rotation restricting mechanism (1, 1B, 1C, 1D, 1E, 1F) comprising a rotation-restricting-mechanism-rotating-body (2, 2B, 2C, 2D, 2F); and a fixed restricting member (8, 33) configured to engage with the rotation-restricting-mechanism-rotating-body (2, 2B, 2C, 2D, 2F) by moving in a rotation axis direction of the rotation-restricting-mechanism-rotating-body (2, 2B, 2C, 2D, 2F) and restrict rotation of the rotation-restricting-mechanism-rotating-body (2, 2B, 2C, 2D, 2F), wherein the rotation-restricting-mechanism-rotating-body (2, 2B, 2C, 2D, 2F) comprises: a plurality of beam-like radial-support parts extending in a radial direction in a radial pattern, inner end portions of the radial-support parts being configured to be connected to a rotator to rotate integrally with the rotator, the inner end portion of each of the radial-support parts being located at an inner end in the radial direction of the radial-support part; a plurality of beam-like first-circumferential-connecting parts connecting the inner end portions of the radial-support parts that are mutually adjacent; a plurality of beam-like second-circumferential-connecting parts connecting outer end portions of the radial-support parts that are mutually adjacent, to thereby form a polygonal shape, the outer end portion of each of the radial-support parts being located at an outer end in the radial direction of the radial-support part; and a plurality of engaging protrusions (7, 31, 32), each of the engaging protrusions (7, 31, 32) extending outwardly in the radial direction from the outer end portion of each of the radial-support parts, wherein in a state where the fixed restricting member (8, 33) is located so as to overlap the engaging protrusion (7, 31, 32) when viewed in a direction orthogonal to the axial direction, an interval between the engaging protrusions (7, 31, 32) that are mutually adjacent in a circumferential direction out of the plurality of engaging protrusions (7, 31, 32) has a distance that enables relative movement of the fixed restricting member (8, 33) in the circumferential direction between the adjacent engaging protrusions (7, 31, 32). a plurality of beam-like radial-support parts extending in a radial direction in a radial pattern, inner end portions of the radial-support parts being configured to be connected to a rotator to rotate integrally with the rotator, the inner end portion of each of the radial-support parts being located at an inner end in the radial direction of the radial-support part; a plurality of beam-like first-circumferential-connecting parts connecting the inner end portions of the radial-support parts that are mutually adjacent; a plurality of beam-like second-circumferential-connecting parts connecting outer end portions of the radial-support parts that are mutually adjacent, to thereby form a polygonal shape, the outer end portion of each of the radial-support parts being located at an outer end in the radial direction of the radial-support part; and a plurality of engaging protrusions (7, 31, 32), each of the engaging protrusions (7, 31, 32) extending outwardly in the radial direction from the outer end portion of each of the radial-support parts, wherein in a state where the fixed restricting member (8, 33) is located so as to overlap the engaging protrusion (7, 31, 32) when viewed in a direction orthogonal to the axial direction, an interval between the engaging protrusions (7, 31, 32) that are mutually adjacent in a circumferential direction out of the plurality of engaging protrusions (7, 31, 32) has a distance that enables relative movement of the fixed restricting member (8, 33) in the circumferential direction between the adjacent engaging protrusions (7, 31, 32).