An object of the invention is to provide a semiconductor device having improved performance, high reliability, and a reduced chip size, in particular, to provide a semiconductor device having an MOSFET over an SOI substrate capable of maintaining its reliability while controlling the potential of a well below a gate electrode and preventing generation of parasitic capacitance. Generation of parasitic capacitance is prevented by controlling the potential of a well below a gate electrode by using a well contact plug passing through a hole portion formed in a gate electrode wiring. Generation of defects in a gate insulating film is prevented by making use of a gettering effect produced by causing an element isolation region to extend along the gate electrode.