Abstract: Disclosed is a semiconductor device having a support region, an element-forming region (e.g., an epitaxial layer) and a buried layer between the support region and the element-forming region, with at least one of a MOS element and a bipolar element being formed in the element-forming region. The feature of the present invention resides in that atoms of at least one element selected from oxygen, nitrogen, carbon, argon, neon, krypton and helium is contained in a layer in at least one of the element-forming region and the buried layer, so as to suppress auto-doping of impurities from the buried layer into the element-forming region and suppress swelling of the buried layer.