PURPOSE: To facilitate formation without necessity for processing non-closely adhering part on a periphery and cylindrical grinding, by mirror-polishing both surface of wafer high in impurity concentration and one low in it, and uniting them closely, and then grinder-lapping a plane of an element-formed side.

CONSTITUTION: N-type impurity low-concentration wafer 11, which composes an element-formed plane in a switching regulator or the like, and N-type impurity high-concentration wafer 12, which serves as a base material, are provided. And, both planes are concurrently polished to turn four planes 11a, 11b, 12a, 12b into mirror planes, with a cleaning process being fully performed on their closely adhering plane. Both wafers are made to closely adhere by close- adhesion equipment. Two closely-adhering wafers 11 and 12, are pressed and made by heat treatment so that a wafer 13 having the closely-adhering plane of wafer strengthened can be obtained. Thus, in a wafer in which close adhesion and strengthening are performed from a 4in-size wafer, for example, bevel etching is performed in the 4in-size and grind-lapping is performed on an element-formed plane 11c to a specified thickness necessary for an element region.