To provide a turning device of an assembly capable of surely controlling the wobbling of the processed face of a disc within an allowable limit in an assembly having a problem of the wobbling of the processed face of the disc at the time of rotating the disc after it is assembled to an attaching object.

An assembly as a turning object is a brake disc unit comprising a bearing, an axle hub, and a disc 16. A shaft-like gripping tool 22 is rotatably installed at the tip of a main spindle 20 as a rotating member, and a movable member 30 is arranged above it. An inner race 11 of the bearing is nipped between an annular lower clamp projection 23 formed at the top end of the shaft-like gripping tool 22 and an annular upper clamp projection 31 formed at the bottom end of the movable member 30 so as to rotatably hold the hub and the disc 16. While the hub and the disc 16 are rotated synchronously with the main spindle 20, the processed face of the disc 16 is turned by a cutting mechanism 14.