Provided is a rotational-force transmitting part including: a rotation support member that is supported, with respect to a first member, so as to be rotatable about a predetermined axis by means of a bearing and that is provided with a first fitting surface formed of a cylindrical outer surface or a cylindrical inner surface centered on the axis; a rotational-force transmitting member that transmits a rotational force with respect to a second member and that is provided with a second fitting surface to be fitted onto the first fitting surface; an annular elastic member that is disposed at such a position as to seal a space between the first fitting surface and the second fitting surface; and a fixing member that fixes the rotational-force transmitting member and the rotation support member in a state in which the first fitting surface and the second fitting surface are fitted.