An optical disk recording medium includes a single layer disk substrate having a hub provided on one side which includes a circular magnetic plate for effecting chucking of the optical disk. On the opposite side of the disk substrate a reinforcing plate is attached for reducing disk warpage during chucking, loading etc. The circular magnetic plate includes a centering opening for disk chucking, while a center opening of the reinforcing plate is larger in diameter than the centering opening. The reinforcing plate covers a central opening formed in the disk substrate and includes a boss portion which is projected into the opening. A groove is provided in the reinforcing plate at the outer circumference of the boss portion for retaining excess adhesive when the substrate is affixed to the reinforcing plate.