A rotation mechanism, comprising: a shaft; and a rotating member (11, 12, 13, 14) having a shaft insertion hole (13b, 14b, 25a, 25b) in which the shaft is inserted, the shaft contacting an inner peripheral surface of the rotating member (11, 12, 13, 14), wherein the rotating member (11, 12, 13, 14) is rotatable relative to the shaft, wherein a surface roughness Ra of the inner peripheral surface is equal to or less than 1.6 μm, and wherein a static friction coefficient of the inner peripheral surface against the shaft is equal to or less than 0.2, wherein a portion of the rotating member (11, 12, 13, 14) that at least includes the inner peripheral surface and surrounds the shaft is made of resin, and wherein the shaft is made of metal, wherein the rotation mechanism further comprising: an internal gear having internal teeth; an oscillating gear (11, 12) having external teeth (23a, 23b) meshing with the internal teeth and being oscillatory rotated; an input crankshaft having an eccentric portion (21a, 21b) that rotatably supports the oscillating gear (11, 12); an output shaft to which a rotational force of the oscillating gear (11, 12) is transmitted; and a support member (13, 14) supporting each axial end portion of the output shaft rotatably, wherein the input crankshaft transmits a rotational force to the oscillating gear (11, 12), wherein the shaft includes the output shaft, wherein the rotating member (11, 12, 13, 14) includes the oscillating gear (11, 12) and the support member (13, 14), wherein the shaft insertion hole (13b, 14b, 25a, 25b) includes a gear-side shaft hole (25a, 25b) formed in the oscillating gear (11, 12) and a support member-side shaft hole (13b, 14b) formed in the support member (13, 14). a shaft; and a rotating member (11, 12, 13, 14) having a shaft insertion hole (13b, 14b, 25a, 25b) in which the shaft is inserted, the shaft contacting an inner peripheral surface of the rotating member (11, 12, 13, 14), wherein the rotating member (11, 12, 13, 14) is rotatable relative to the shaft, wherein a surface roughness Ra of the inner peripheral surface is equal to or less than 1.6 μm, and wherein a static friction coefficient of the inner peripheral surface against the shaft is equal to or less than 0.2, wherein a portion of the rotating member (11, 12, 13, 14) that at least includes the inner peripheral surface and surrounds the shaft is made of resin, and wherein the shaft is made of metal, wherein the rotation mechanism further comprising: an internal gear having internal teeth; an oscillating gear (11, 12) having external teeth (23a, 23b) meshing with the internal teeth and being oscillatory rotated; an input crankshaft having an eccentric portion (21a, 21b) that rotatably supports the oscillating gear (11, 12); an output shaft to which a rotational force of the oscillating gear (11, 12) is transmitted; and a support member (13, 14) supporting each axial end portion of the output shaft rotatably, wherein the input crankshaft transmits a rotational force to the oscillating gear (11, 12), wherein the shaft includes the output shaft, wherein the rotating member (11, 12, 13, 14) includes the oscillating gear (11, 12) and the support member (13, 14), wherein the shaft insertion hole (13b, 14b, 25a, 25b) includes a gear-side shaft hole (25a, 25b) formed in the oscillating gear (11, 12) and a support member-side shaft hole (13b, 14b) formed in the support member (13, 14). an internal gear having internal teeth; an oscillating gear (11, 12) having external teeth (23a, 23b) meshing with the internal teeth and being oscillatory rotated; an input crankshaft having an eccentric portion (21a, 21b) that rotatably supports the oscillating gear (11, 12); an output shaft to which a rotational force of the oscillating gear (11, 12) is transmitted; and a support member (13, 14) supporting each axial end portion of the output shaft rotatably, wherein the input crankshaft transmits a rotational force to the oscillating gear (11, 12), wherein the shaft includes the output shaft, wherein the rotating member (11, 12, 13, 14) includes the oscillating gear (11, 12) and the support member (13, 14), wherein the shaft insertion hole (13b, 14b, 25a, 25b) includes a gear-side shaft hole (25a, 25b) formed in the oscillating gear (11, 12) and a support member-side shaft hole (13b, 14b) formed in the support member (13, 14).