A method for producing a semiconductor device with an SOI substrate having a support substrate 1 and a semiconductor layer 3 that interpose a first insulating film 2 between them includes the following steps. An element region and an element-separation region 4 are formed in the semiconductor layer 3. A gate insulating film 5 is formed on the semiconductor layer 3. A gate electrode 6 is formed on the gate insulating film 5. A second insulating film 7 is formed. The gate insulating film 5 is removed. First thickness adjustment is performed. Ion implantation introducing low concentration impurities is performed on the thickness-adjusted semiconductor layers 3 and 8. A first sidewall portion 7a is formed on the side surfaces of the gate electrode 6. A second sidewall portion 10a is formed on the side surfaces of the first sidewall portion 7a.