A speed reduction mechanism comprising a first gear and a second gear , and comprising: one first tooth part provided on the first gear and spirally extending in an axial direction of the first gear ; an engagement projected part that is provided on the first tooth part , has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear , formed in an arc shape, and has a curvature center (C2) provided at a position eccentric from a rotation center (C1) of the first gear ; a plurality of second tooth parts that are provided on the second gear , are inclined with respect to the axial direction of the first gear , and are arranged in a circumferential direction of the second gear ; and an engagement recessed part that is provided between adjacent second tooth parts , has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear , formed in an arc shape, and is engaged with the engagement projected part , the speed reduction mechanism being characterized in that when the second gear is viewed from the axial direction of the first gear , a shape of each the plurality of second tooth parts is in a curve shape that is shifted from an operation locus (Sep) of the curvature center (C2) of the engagement projected part toward a radially inner side of the second gear 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 (P) arranged on a most radially inner side of the second gear among points that form the operation locus (Sep) of the curvature center (C2) of the engagement projected part . one first tooth part provided on the first gear and spirally extending in an axial direction of the first gear ; an engagement projected part that is provided on the first tooth part , has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear , formed in an arc shape, and has a curvature center (C2) provided at a position eccentric from a rotation center (C1) of the first gear ; a plurality of second tooth parts that are provided on the second gear , are inclined with respect to the axial direction of the first gear , and are arranged in a circumferential direction of the second gear ; and an engagement recessed part that is provided between adjacent second tooth parts , has a cross-sectional shape, which is along a direction orthogonal to the axial direction of the first gear , formed in an arc shape, and is engaged with the engagement projected part , the speed reduction mechanism being characterized in that when the second gear is viewed from the axial direction of the first gear , a shape of each the plurality of second tooth parts is in a curve shape that is shifted from an operation locus (Sep) of the curvature center (C2) of the engagement projected part toward a radially inner side of the second gear 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 (P) arranged on a most radially inner side of the second gear among points that form the operation locus (Sep) of the curvature center (C2) of the engagement projected part . <EOS>