Segmental outward projections (12) are provided at radially opposite positions on the circumference of a tubular mount portion (7) of a main body (5), and truncated side portions (13) are provided between and at the opposite sides of the segmental outward projection (12). On the other hand, D-Shaped inward projections (14) are projected radially inward from a boundary portion between fore and rear inner peripheral surface portions (10B) and (10C) of a shaping air ring (10) in such a way as to define an outward projection passageway (15) conforming in shape with the segmental outward projections (12). At the time of mounting the rotary atomizing head (4) on the rotational shaft (3), the outward projections (12) are put in a conforming angular position relative to the outward projection passageway (15) to pass the inward projections (14). In the event the rotary atomizing head (4) is loosened and its position is shifted in a forward direction relative to the rotational shaft (3), the outward projections(12) are abutted against the inward projections (14) to prevent the rotary atomizing head (4) from falling off the rotational shaft (3).