A speed reduction mechanism (30, 50, 60, 80, 90, 320, 400) comprising a first gear (31, 92, 330) and a second gear (32, 81, 91, 340), and comprising: one first tooth part (31b, 51, 61) provided on the first gear (31, 92, 330) and spirally extending in an axial direction of the first gear (31, 92, 330); an engagement projected part that is provided on the first tooth part (31b, 51, 61), has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear (31, 92, 330), formed in an arc shape, and has a curvature center provided at a position eccentric from a rotation center of the first gear (31, 92, 330); a plurality of second tooth parts (32c, 91a) that are provided on the second gear (32, 81, 91, 340), are inclined with respect to the axial direction of the first gear (31, 92, 330), and are arranged in a circumferential direction of the second gear (32, 81, 91, 340); and an engagement recessed part that is provided between adjacent second tooth parts (32c, 91a), has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear (31, 92, 330), formed in an arc shape, and is engaged with the engagement projected part, the speed reduction mechanism (30, 50, 60, 80, 90, 320, 400) being characterized in that when the second gear (32, 81, 91, 340) is viewed from the axial direction of the first gear (31, 92, 330), a shape of each the plurality of second tooth parts (32c, 91a) is in a curve shape that is shifted from an operation locus (Sep) of the curvature center of the engagement projected part toward a radially inner side of the second gear (32, 81, 91, 340) by a curvature radius (r) of the engagement projected part, and a shape of the engagement recessed part is in an arc shape having a radius that is the curvature radius (r) of the engagement projected part and having a center that is a point arranged on a most radially inner side of the second gear (32, 81, 91, 340) among points that form the operation locus (Sep) of the curvature center of the engagement projected part. one first tooth part (31b, 51, 61) provided on the first gear (31, 92, 330) and spirally extending in an axial direction of the first gear (31, 92, 330); an engagement projected part that is provided on the first tooth part (31b, 51, 61), has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear (31, 92, 330), formed in an arc shape, and has a curvature center provided at a position eccentric from a rotation center of the first gear (31, 92, 330); a plurality of second tooth parts (32c, 91a) that are provided on the second gear (32, 81, 91, 340), are inclined with respect to the axial direction of the first gear (31, 92, 330), and are arranged in a circumferential direction of the second gear (32, 81, 91, 340); and an engagement recessed part that is provided between adjacent second tooth parts (32c, 91a), has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear (31, 92, 330), formed in an arc shape, and is engaged with the engagement projected part, the speed reduction mechanism (30, 50, 60, 80, 90, 320, 400) being characterized in that when the second gear (32, 81, 91, 340) is viewed from the axial direction of the first gear (31, 92, 330), a shape of each the plurality of second tooth parts (32c, 91a) is in a curve shape that is shifted from an operation locus (Sep) of the curvature center of the engagement projected part toward a radially inner side of the second gear (32, 81, 91, 340) by a curvature radius (r) of the engagement projected part, and a shape of the engagement recessed part is in an arc shape having a radius that is the curvature radius (r) of the engagement projected part and having a center that is a point arranged on a most radially inner side of the second gear (32, 81, 91, 340) among points that form the operation locus (Sep) of the curvature center of the engagement projected part.