Method of manufacturing semiconductor substrate dielectric isolating structure

A manufacturing method of this invention improves nonuniformity in film thickness of a circuit element formation region produced due to a poor flatness of a semiconductor substrate in the manufacture of a semiconductor substrate having a dielectric isolating structure. Mirror-polished surfaces of first and second semiconductor substrates are opposed and bonded to each other so as to sandwich a dielectric having a predetermined thickness, and the first semiconductor substrate is ground from the surface opposite to the adhesion surface to have a predetermined thickness with reference to the dielectric. An impurity is doped in the first semiconductor substrate to form a high-concentration impurity layer having an impurity concentration corresponding to a predetermined low-concentration impurity layer having a predetermined thickness thereon, thereby constituting a circuit element region. This invention is a method of manufacturing a semiconductor substrate, which improves film thickness precision of each circuit element formation layer for forming a circuit element.