A conventional semiconductor device has a problem that an on-current of a parasitic transistor flows through a surface portion of a semiconductor layer and thus a semiconductor element undergoes thermal breakdown. In a semiconductor device according to the present invention, a protection element is formed with use of an isolation region and N type buried layers. A PN junction region in the protection element is formed on a P type buried layer of the isolation region. The PN junction region has a junction breakdown voltage lower than that of a PN junction region of a semiconductor element to be protected. This structure allows an on-current of a parasitic transistor to flow into the protection element, and thereby the semiconductor element is protected. In addition, the on-current of the parasitic transistor flows through a deep portion of the epitaxial layer, and thereby the protection element is prevented from thermal breakdown.