A semiconductor substrate that has a MOS transistor with a high breakdown voltage having double sidewall insulation films and can inhibit negative effects on the electric characteristics and method thereof. The semiconductor device is formed as a transistor with a configuration having gate insulation film 21 and gate electrode 22 formed on semiconductor substrate 10, inner sidewall insulation film 25 formed at least on part of the gate insulation film and on both sides of the gate electrode, outer sidewall insulation film 26 formed at least on part of the gate insulation film and on both sides of the inner sidewall insulation film, low concentration impurity area 23 containing an impurity at a low concentration and formed in the semiconductor substrate in the area underneath the inner sidewall insulation film and the outer sidewall insulation film, and high concentration impurity area 27 containing an impurity at a concentration higher than the low concentration impurity area and formed in the semiconductor substrate in the area underneath both sides of the outer sidewall insulation film.