To positively grip a deformed workpiece with simple work without changing a jaw (a mouthpiece) mounted to a jaw of a stock vice regarding an attachment used in holding the deformed workpiece with the stock vice.

This attachment provided with two opposed soft jaws is inserted between jaws to grip the workpeice. Two soft jaws are energized in the separating directions from each other by built-in compression spring elements 7 and provided with stopper elements for locking the clearance. The basic shape of the soft jaw is a rectangular solid, and a recess corresponding to the outline of the workpiece is machined for use. The side face of the soft jaw is provided with a dislocation preventing element projected onto the back face side. A reference dislocation preventing element and an opposed movable dislocation preventing element are provided as the dislocation preventing element to accurately determine the machining reference of the workpiece.