PURPOSE: To enable the univocal determination of the front and the rear surfaces and the orientation, by providing a step part to determine the surface position of a substrate, on the inner periphery of a fixing hole into which a circular plate type glass wafer substrate is engaged, two or more chords spaced different distances away from the center being cut off from said glass wafer substrate.

CONSTITUTION: In a circular plate type glass wafer, two chords 1b, 1c spaced different distances away from the center are cut off so as to cross perpendicularly. An opaque film 2 is formed on said glass substrate. A substrate retaining device 3 is provided with the following on the inner peripheral part; a step part 3a to regulate the position of the surface of the glass wafer 1, and a fixing hole 3c in which two chord-parts 3d, 3e to regulate the orientation and position of the front and the rear surface are formed. Thereby, the confirmation of the front and the rear surfaces by using the figure of the opaque film formed on the glass wafer substrate surface is unnecessitated, and the correct mounting is enabled by only engaging the wafer in the manner in which the shape is matched with the fixing hole of the substrate retaining device. Therefore, the mounting is facilitated, and the operating efficiency is remarkably improved.