PURPOSE: To protect a function section for a semiconductor integrated circuit from both positive and negative electrical impulse inputs by forming a reverse conduction type second region in one region of one conduction type first region and each coupling the residual first region and the second region in the first region with an output terminal and an input terminal.

CONSTITUTION: Constitution in which a transistor 1 is connected between an IC input/output terminal 5 and a grounding terminal 4 is formed. A layer such as an N type epitaxial layer 24 is shaped on a P type substrate 23 through an N type buried layer 26. The epitaxial layer is surrounded by a P type isolation diffusion layer 25, a P type diffusion layer 28 is shaped in a region surrounded by the layer 25 and N type diffusion layers 27, 29 on said P type diffusion layer 28 and the epitaxial layer 24 respectively, and a grounding terminal 4 and an IC input/output terminal 5 are each formed. When an electrical impulse at positive potential is applied, an IC internal circuit is protected by flowing currents through the grounding terminal 4 from the IC input/output terminal 5 by breakdown currents flowing through a transistor 21.