To provide a method for flattening a semiconductor substrate capable of assuring a quality of the surface by highly flattening.

The method for flattening the semiconductor substrate comprises a vapor phase etching step with a plasma of etching a rear surface side of a semiconductor substrate of not a surface to be formed with a device, a polishing step with an infinitesimal processing amount after etching of polishing the substrate, a finishing polishing step of polishing the surface to be formed with the device by inverting the substrate, that is, the front surface of the semiconductor substrate. Thus, the substrate having a high flatness is obtained, the vapor phase etching surface holds a clarity by polishing an infinitesimal amount. Further, the front surface side of the substrate can obtain the surface by polishing as prior art with the high flatness as a reference.