To obtain a thin glass substrate having high flatness without scratches by suspending or dispersing an inorg. powder in a solvent which does not produce a decomposed gas to prepare a soln., applying the prepared soln. on the surface of a sheet glass formed by down loading method, laminating the glass sheets, interposing the glass sheets between compressing plates with high flatness, and heating and slowly cooling the sheets while pressurizing.

The inorg. powder to be applied preferably consists of MgCO3 and/or CaCO3 and contains boron nitride. The particle size is uniform and preferably about 20 to 30 μm. The powder is uniformly applied preferably by rotating a sponge roller impregnated with a soln. in which the inorg. material is suspended along the surface of the sheet glass. The concn. of the soln. is preferably controlled to a low level so as to form a single molecular layer. Moreover, the sponge roller impregnated with water is preferably brought into contact with the sheet glass under pressure. By this flattening method of sheet glass, a succeeding polishing process can be simplified.