PURPOSE: To obtain high reliability by using a transistor of a gate insulating film, which is thicker at least for one part or more than an inner circuit, for an input protective circuit for protecting the inner circuit from the overcurrents and the overvoltage from outside.

CONSTITUTION: An oxide film 7 for element isolation is formed on a substrate 3, and next a first gate oxide film 7 of A in thickness is formed at a desired position. Next, a second gate oxide film 8 of B in thickness is formed, and the thickness is made thicker than that of the film 7. After coverage with gate material, a gate electrode 6 is formed in a desired shaped, and then diffusion layer regions 4 and 5 are formed, and after formations of a through hole 9 and metallic wiring 10, a protective film is overlayed. Hereby, if the gate oxide film on the drain side, to which surge is applied directly, is thickened and that on the source side is kept thin as it is, it can maintain enough distance to the gate electrode while maintaining essential function, whereby the surge breakdown strength can be improved.