PURPOSE: To enlarge the allowable range for displacement to ensure safe and effective working by forming the bottom surface of a groove as the flat surface which is a little wider than the plate thickness of a disk substrate and the side surface as a tapered surface and forming the groove in such a radius of curvature as is a little larger than the radius of the disk substrate.

CONSTITUTION: A groove 3 is formed with a radius of curvature being set a little larger than the radius of a disk substrate 4 by forming the bottom surface of groove 3 as a flat surface 31 which is a little wider than the plate thickness (d) of the disk substrate 4 and the side surface thereof as a tapered surface 32. Thereby, a certain allowance of positioning accuracy for the handling operation can be obtained between a substrate fitting hole 2 of a holder and the disk substrate 4 and safe and easy working can be ensured even if the disk substrate 4 is a little displaced for the fitting hole 2 at the time of loading or unloading the disk substrate 4.