Glass blank, method for producing glass blank, and method for producing glass substrate of magnetic disc

This disc-like glass blank used for producing a glass substrate of a magnetic disc has a thickness at a center and a perimeter thereof that is greater than the thickness at an intermediate portion ina radial direction, and when the difference between the maximum blank thickness and the minimum blank thickness between both principle surfaces of the glass blank is defined as D and the length of a line segment on a principle surface connecting a position where the perimeter has the maximum blank thickness and the center position of the glass blank is defined as R, the difference between the thickness at a position located away from the center position on the line segment by a distance of 0.9R and the maximum blank thickness at the perimeter is greater than 0.2D. 用于制造磁盘用玻璃基板的圆板状的玻璃坯料的中央部和外周部厚于半径方向的中间部，将玻璃坯料的两主表面间的最大板厚与最小板厚之差设为D，将连结外周部的达到最大板厚的位置与上述玻璃坯料的中心位置的主表面上的线段的长度设为R时，线段上的自中心位置起为0.9R的距离的位置处的板厚与外周部的最大板厚的板厚差大于0.2D。