PURPOSE: To obtain a flat disk substrate superior in flatness, by peeling a non- magnetic nickel-phosphorus plating film formed on a supporting body whose surface is smooth, placing it in a molding die, and integrating it with a resin in the die under heating.

CONSTITUTION: A plating film forming supporting body 1 has a disk-shaped recessed part so as to form the plating film in a disk shape, and the lower plane 11 forms a flat and ground specular plane. A masking 9 is applied at the peripheral plane of the recessed part of the supporting body 1, and a nickel- phosphorus alloy film 2 is formed at the lower plane 11 by an electroplating, or an electroless plating, and the plating film 2 is peeled from the surface of the supporting body 1, and it is placed in the inner plane of a disk substrate molding die 3. At this time, the smooth plane (the plane coming in contact with the supporting body 1) or the plating film 2 is opposed to the inner plane of the die 3. Next, a synthesis resin 4 is supplied and filled in the die 3, and the plating film 2, and the resin 4 are integrated under heating. After integration, it is cooled maintaining pressurization, and the disk substrate is taken out, and immediately after that, a magnetic layer is formed on the plating film 2.