A windmill comprising: a base-side structure (102, 104); a rotation-side structure (103, 105) that is rotatable with respect to the base-side structure (102, 104); and a rotation driving mechanism for the windmill (1,1a,1b) that rotates the rotation-side structure (103, 105) with respect to the base-side structure (102, 104), the rotation driving mechanism for windmill (1, 1a, 1b) comprising: an annular track part (2, 22, 2a, 2b) provided in one structure of the base-side structure (102, 104) and the rotation-side structure (103, 105), the annular track part (2, 22, 2a, 2b) having a circumferential wall part (4, 24), and first teeth (7, 7a) that are arranged along a circumferential direction of the circumferential wall part (4, 24) such that a facewidth direction of each first tooth (7, 7a) extends along a direction in parallel with a central axis direction of the circumferential wall part (4, 24); a rotation driving part having a rotatable rotating shaft, the rotation driving part being fixed on the other structure of the base-side structure (102, 104) and the rotation-side structure (103, 105); and characterized in that the rotation driving mechanism for windmill further comprises a swinging unit including swinging parts (15, 15a, 15b) each having a meshing part to be meshed with a part of the first teeth (7, 7a), and a swinging part body (16a, 16c) provided with the meshing part, wherein the swinging parts (15, 15a, 15b) are arranged along the facewidth direction of the first teeth (7, 7a), and the swinging unit is configured to be relatively moved with respect to the annular track part (2, 22, 2a, 2b) in a circumferential direction of the annular track part (2, 22, 2a, 2b), when the rotating shaft is rotated so that the swinging parts (15, 15a, 15b) are swung with maintaining a predetermined phase difference thereamong, wherein the meshing part includes second teeth to be meshed with the first teeth (7, 7a). a base-side structure (102, 104); a rotation-side structure (103, 105) that is rotatable with respect to the base-side structure (102, 104); and a rotation driving mechanism for the windmill (1,1a,1b) that rotates the rotation-side structure (103, 105) with respect to the base-side structure (102, 104), the rotation driving mechanism for windmill (1, 1a, 1b) comprising: an annular track part (2, 22, 2a, 2b) provided in one structure of the base-side structure (102, 104) and the rotation-side structure (103, 105), the annular track part (2, 22, 2a, 2b) having a circumferential wall part (4, 24), and first teeth (7, 7a) that are arranged along a circumferential direction of the circumferential wall part (4, 24) such that a facewidth direction of each first tooth (7, 7a) extends along a direction in parallel with a central axis direction of the circumferential wall part (4, 24); a rotation driving part having a rotatable rotating shaft, the rotation driving part being fixed on the other structure of the base-side structure (102, 104) and the rotation-side structure (103, 105); and characterized in that the rotation driving mechanism for windmill further comprises a swinging unit including swinging parts (15, 15a, 15b) each having a meshing part to be meshed with a part of the first teeth (7, 7a), and a swinging part body (16a, 16c) provided with the meshing part, wherein the swinging parts (15, 15a, 15b) are arranged along the facewidth direction of the first teeth (7, 7a), and the swinging unit is configured to be relatively moved with respect to the annular track part (2, 22, 2a, 2b) in a circumferential direction of the annular track part (2, 22, 2a, 2b), when the rotating shaft is rotated so that the swinging parts (15, 15a, 15b) are swung with maintaining a predetermined phase difference thereamong, wherein the meshing part includes second teeth to be meshed with the first teeth (7, 7a). an annular track part (2, 22, 2a, 2b) provided in one structure of the base-side structure (102, 104) and the rotation-side structure (103, 105), the annular track part (2, 22, 2a, 2b) having a circumferential wall part (4, 24), and first teeth (7, 7a) that are arranged along a circumferential direction of the circumferential wall part (4, 24) such that a facewidth direction of each first tooth (7, 7a) extends along a direction in parallel with a central axis direction of the circumferential wall part (4, 24); a rotation driving part having a rotatable rotating shaft, the rotation driving part being fixed on the other structure of the base-side structure (102, 104) and the rotation-side structure (103, 105); and characterized in that the rotation driving mechanism for windmill further comprises a swinging unit including swinging parts (15, 15a, 15b) each having a meshing part to be meshed with a part of the first teeth (7, 7a), and a swinging part body (16a, 16c) provided with the meshing part, wherein the swinging parts (15, 15a, 15b) are arranged along the facewidth direction of the first teeth (7, 7a), and the swinging unit is configured to be relatively moved with respect to the annular track part (2, 22, 2a, 2b) in a circumferential direction of the annular track part (2, 22, 2a, 2b), when the rotating shaft is rotated so that the swinging parts (15, 15a, 15b) are swung with maintaining a predetermined phase difference thereamong, wherein the meshing part includes second teeth to be meshed with the first teeth (7, 7a).