Disk-shaped glass plate and magnetic disk

A disk-shaped glass plate,

• the disk-shaped glass plate being a glass substrate for a magnetic disk or a material for a glass substrate for a magnetic disk,
• the disk-shaped glass plate having a circular hole at a center position of the disk-shaped glass plate,
• the disk-shaped glass plate having a pair of main surfaces, an outer peripheral edge surface, and an inner peripheral edge surface along the circular hole, and
• a difference of a maximum retardation value and a minimum retardation value among retardation values of twelve measurement points in the at least one of the pair of main surfaces of the disk-shaped glass plate being 30 nm or less, the twelve measurement points including four points on a circumference that are located away from each other at 90 degrees at each of three radial positions that are located radially outward away from an inner circumferential end by 25%, 50%, and 75% in the at least one of the pair of main surfaces of which the inner circumferential end is set to be 0% and of which an outer circumferential end is set to be 100%.