the face side and projected portions of the back side are processed and removed so that the height at each point in the back side is adjusted to the height of the most recessed portion in the back side to thereby achieve flattening of the back side for example where the processing tool is a sandblast nozzle the method of processing for correction of parallelness and correction of flatness can be carried out by use of a device shown in fig3 here numeral 10 in the figure denotes a substrate holding base 11 denotes a sandblast nozzle and 12 denotes an airflow containing abrasive grains numeral 1 denotes the substrate the processing tool has such a structure as to be arbitrarily movable in x and y directions and the movement can be controlled by a computer in addition the processing can also be performed by an x u03b8 mechanism the air pressure has relations with the abrasive grains used and the distance between the processing tool and the substrate cannot be determined univocally and can be regulated in view of removal speed and processing strain depth in addition in this production method only the projected portions and thick portions of the substrate are selectively removed so that a substrate poor in flatness can be securely improved and it is possible not only to obtain a high flatness substrate by precision control of the processing tool but also to speedily realize an improvement in the flatness of the substrate by a rough control of the processing tool the abrasive grains to be used are not particularly restricted and are preferably those of 600 to 3000 where the grain diameter of the abrasive grains is greater than 600 the strain due to processing is large so that the processing.