A rotation driving mechanism for windmill (1) includes an annular track part (2), a rotation driving part (11), and a plurality of swinging parts (15). The annular track part (2) is disposed on one of a base-side structure and a rotation-side structure, and has a circumferential wall part (4) and first teeth (7). The rotation driving part (11) is fixed on the other of the base-side structure and the rotation-side structure. Each swinging part (15) has a swinging part body (16a) and second teeth (16b). When a rotating shaft (13) of the rotation driving part (11) is rotated so that the swinging parts (15) are swung with maintaining a predetermined phase difference thereamong, the swinging parts (15) are relatively moved with respect to the annular track part (2).