A semiconductor device includes: a first transistor and a second transistor disposed in or on a silicon substrate; an element isolation structure that isolates the first transistor and the second transistor, the element isolation structure comprising at least one of a first element isolation film disposed in a region of a first well disposed in a formation area of the first transistor, or a second element isolation film disposed in a region of a second well disposed in a formation area of the second transistor, and a third well disposed under the first well in the silicon substrate and is electrically connected to the second well. The first element isolation film or the second element isolation film has a portion that does not extend over a boundary between the first well and the second well.