To reduce the number of processes for forming a plurality of transistor areas whose breakdown voltages are different.

An N channel high breakdown voltage transistor area, a P channel type high breakdown voltage transistor area and an N channel low breakdown voltage transistor area are formed with a first oxide film 3 and an LOCOS oxide film 5, and both breakdown voltage transistor areas are formed with second oxide films 7A1, 7A2 and 7B thicker than the first oxide film 3 so that high breakdown voltages can be maintained, and N+ type source/drain areas 23 and 24 are simultaneously formed. Also, the P channel type high breakdown transistor area is formed with a second oxide film 7B so that the protection of an offset part can be attained.