A semiconductor integrated circuit device having an SOI structure is capable of preventing occurrence of leak current flowing from a diffusion layer even when a semiconductor element having a pn-junction is included in the semiconductor substrate. The semiconductor integrated circuit device having the SOI structure is formed with a semiconductor layer, or SOI layer, on a p-type semiconductor substrate through a buried insulating film and further with semiconductor circuit elements serving as functional elements at the SOI layer thus formed. As a protection transistor to protect the semiconductor circuit elements, a MOSFET may be formed in which n-type diffusion layers are formed in the semiconductor substrate. The n-type diffusion layers of the MOSFET are to be surrounded by p-type diffusion layers more highly doped than the semiconductor substrate.