To manufacture and process a T-die, having a mirror surface with Rx of about 0.01 μm and Ry of about 0.10 μm and having a length of 5 to 10 m.

Coolant liquid is supplied, through a pair of workpiece surface coolant liquid supply pipes 20a and 20b attached to the front and back of the machine casing 6 of a plane grinding device, to the surface of a magnetic chuck 13 for always fixing a workpiece w to form a coolant liquid film on the entire area of the surface of the magnetic chuck, and the temperature of the entire area of the workpiece is maintained constant, and grinding process is carried out at two stages or more by a diamond grinding wheel 3.