The reliability of a semiconductor device including a MOSFET formed over an SOI substrate is improved. A manufacturing method of the semiconductor device is simplified. A semiconductor device with n-channel MOSFETsQn formed over an SOI substrate SB includes an n+-type semiconductor region formed as a diffusion layer over an upper surface of a support substrate under a BOX film, and a contact plug CT2 electrically coupled to the n+-type semiconductor region and penetrating an element isolation region, which can control the potential of the support substrate. At a plane of the SOI substrate SB, the n-channel MOSFETsQn each extend in a first direction, and are arranged between the contact plugs CT2 formed adjacent to each other in the first direction.