A rotation mechanism includes a casing with an upper half portion and a lower half portion; and an internal unit disposed in the casing with a rotor rotating around an axis thereof. Additionally, a bearing portion rotatably supports the rotor, and an annular seal portion seals a gap surrounding a circumferential surface of the rotor. An axial movement restricting portion includes a fitting concave portion on one of the casing and the internal unit, and a fitting convex portion to be fitted into the fitting concave portion as a pair, which restricts relative movement between the casing and the internal unit in a direction of axis. A tapered surface is formed on the fitting concave portion and the fitting convex portion and a width thereof in the direction of axis increases toward an inner circumferential side in a radial direction.