PURPOSE: To improve the parallelism and workability of a substrate by interposing plural spherical bodies having an identical radius between an inner circumferential region and an outer circumferential region of two discoid substrates having a hole at the center and bonding the bodies by means of an adhesives.

CONSTITUTION: The plural spherical bodies (e.g., glass balls) 13W16 having identical radiuses are arranged to the inner circumferential region and the outer circumferential region of the two circular transparent substrates 1, 2 having holes 3, 4 at the center together with adhesives 17W20 to bond the substrates 1 and 2. Since the spherical bodies 13W16 are arranged scatteringly, the parallelism and workability of the substrates 1, 2 are improved.