PURPOSE: To prevent the occurrence of bubbles by a method in which the diameter of the central hole of a plastic base plate is made smaller than the diameter of the central hole of the complete object of an optical disk, and the relation between the diameter of a boss and the diameter of the central hole of the plastic base plate is prescribed.

CONSTITUTION: The diameter of the central hole of a plastic base plate is made smaller than the diameter of the central hole of the complete object of an optical disk, and the relation between the diameter r1 of a boss 8 and the diameter r2 of the central hole of the plastic base plate is kept in r2-5mm≤r1≤r2-0.3. Consequently, when the base plate is placed, the clearance 10 generated between a central boss 8 and the inner peripheral part of the central hole of the base plate, has the size of some degree, therefore since the bubble 9 which remained in the clearance is relatively large, the bubble stays in the central hole, and even when the bubble is generated on the inner peripheral surface of the base plate 4, the bubble is moved to inner peripheral part and stays in the clearance 10 when the resin which has been pressed extends. When the clearance is less than 0.3mm, small bubbles are generated easily as usual, and when it is larger than 5mm, the base plate moves easily when it is pressed, whereby the stamper 1 may receive damage by the contact with the stamper 1 etc.