A semiconductor device that suppresses variation and a drop in the breakdown voltage of transistors. In the semiconductor device in which a logic transistor and a high-breakdown-voltage transistor are formed on one Si substrate, an insulating film which has an opening region and which is thick around the opening region is formed on a low concentration drain region formed in the Si substrate on one side of a gate electrode of the high-breakdown-voltage transistor. The insulating film around the opening region has a two-layer structure including a gate insulating film and a sidewall insulating film. When ion implantation is performed on the low concentration drain region beneath the opening region to form a high concentration drain region, the insulating film around the opening region prevents impurities from passing through. This eliminates variation in the relative positions of the opening region and a place where the high concentration drain region is formed, and the high concentration drain region can be formed on a self align basis with respect to the low concentration drain region.