To simplify the finish working of a dental prosthesis workpiece.

This prosthesis workpiece 1 has a body 2 with an unworked end extending to a cylindrical plug 3. The plug 3 includes flat reference faces 12, 14 opposing each other while, in symmetry with respect to the axis 5 of the workpiece 1 and being parallel to the axis 5, a semicircular slit bottom 15 extending inward of the reference faces 12, 14, and spread guide face parts 11, 13 extending outward of the reference faces. The interval between the reference faces 12, 14 is equal to the outside diameter of the opposite reference element of a collet, and when the plug 3 is fitted into the collet, the opposite reference element abuts on the reference faces 12, 14 and the slit bottom 15 to position the workpiece 1 both circumferentially and axially.