Workpiece unit and producing method thereof

A workpiece unit, comprising:

• a workpiece body;
• and
• a holding member having an inner circumferential portion that directly contacts over an entire circumference on at least one part of a circumferential portion of the workpiece body,
• wherein:
• the workpiece body itself comprises at least one first positioning portion which is integral with and formed from the same material as the workpiece body and is arranged over an entire circumference of the workpiece body and at a part facing the holding member,
• the holding member itself comprises a second positioning portion engaged with the first positioning portion over an entire circumference of the workpiece body,
• the holding member is made of plastic,
• information including any of a lot number, a color or shade, a shrinkage ratio and a discrimination for upper/lower sides is given to the holding member,
• the holding member is held by a machining apparatus,
• the workpiece body comprises a first surface, a second surface on an opposite side to the first surface, and a circumferential portion arranged between the first surface and the second surface,
• the circumferential portion and the inner circumferential portion are not oriented in the same direction as either of the first surface and the second surface,
• the workpiece body comprises a projecting portion projecting from the circumferential portion,
• the first positioning portion is arranged on the projecting portion, and
• the holding member is arranged in at least a projection direction of the projecting portion.