A speed reducing mechanism (1A, 1B, 1C), comprising: a case configured to output rotation to a mating member from a drive source (107, 108, 109), which is producing a rotational force around an axis; at least one crankshaft provided in the case and configured to rotate by receiving the rotation of the drive source (107, 108, 109); a carrier provided in the case and configured to decelerate rotation of the crankshaft; a coupling member configured to couple the mating member and the case by elastic deformation; and an anti-rotation portion configured to prevent relative rotation between the mating member and the case, wherein the case is fitted into a through hole formed to pass through the mating member in an axial direction along the axis, and an outer periphery of the case is in contact with an inner surface of the through hole, the coupling member includes: an elastic deformation portion extending in the axial direction along the axis and capable of elastic deformation in a radial direction intersecting the axial direction; and a claw portion provided at a distal end of the elastic deformation portion and projecting in the radial direction, the coupling member couples the case fitted into the through hole in the axial direction against the mating member, in a state in which the coupling member couples the case and the mating member, the elastic deformation portion extends through the through hole along the axial direction, and the elastic deformation portion is in contact with the inner surface of the through hole by elastic deformation, the anti-rotation portion is provided between the outer periphery of the case and the inner surface of the through hole, and the case is prevented from rotating relative to the through hole in a circumferential direction around the axis by the anti-rotation portion. a case configured to output rotation to a mating member from a drive source (107, 108, 109), which is producing a rotational force around an axis; at least one crankshaft provided in the case and configured to rotate by receiving the rotation of the drive source (107, 108, 109); a carrier provided in the case and configured to decelerate rotation of the crankshaft; a coupling member configured to couple the mating member and the case by elastic deformation; and an anti-rotation portion configured to prevent relative rotation between the mating member and the case, wherein the case is fitted into a through hole formed to pass through the mating member in an axial direction along the axis, and an outer periphery of the case is in contact with an inner surface of the through hole, the coupling member includes: an elastic deformation portion extending in the axial direction along the axis and capable of elastic deformation in a radial direction intersecting the axial direction; and a claw portion provided at a distal end of the elastic deformation portion and projecting in the radial direction, an elastic deformation portion extending in the axial direction along the axis and capable of elastic deformation in a radial direction intersecting the axial direction; and a claw portion provided at a distal end of the elastic deformation portion and projecting in the radial direction, the coupling member couples the case fitted into the through hole in the axial direction against the mating member, in a state in which the coupling member couples the case and the mating member, the elastic deformation portion extends through the through hole along the axial direction, and the elastic deformation portion is in contact with the inner surface of the through hole by elastic deformation, the anti-rotation portion is provided between the outer periphery of the case and the inner surface of the through hole, and the case is prevented from rotating relative to the through hole in a circumferential direction around the axis by the anti-rotation portion.