PURPOSE: To clamp even a thin workpiece prevented from springing to the outside in an accurately positioning condition, by interposing a clamp assist member between an opposed surface of each opposed plate and a clamped end face of the workpiece.

CONSTITUTION: The first curved surface part 71 of a clamp assist member 7 is brought into external contact with a clamped end face of a workpiece W, and the second curved surface part 72 is brought into external contact with a side of an opposed surface 51 of an opposed plate 5. Here by providing a contact part (tangent P part) of the second curved surface part 72 in a point end side of the opposed surface 51 from a contact part (tangent Q part) of the first curved surface part 71, the clamp assist member 7 receives turn force in a direction of void arrow mark with a prescribed rotational center serving as the center, to generate a condition that the first curved surface part 71 is pressed into an opposite side to a point end side of the opposed surface 51. Accordingly, frictional force, generated between the first curved surface part 71 and the clamped end face of the workpiece W, holds the workpiece W while pressed in a direction (lower side) separating distant from the point end side of the opposed surface 51.