A semiconductor device according to one embodiment includes: a semiconductor substrate; an element isolation insulating film embedded in the vicinity of a front surface of the semiconductor substrate; a through plug penetrating the semiconductor substrate from a back surface to the front surface so as to penetrate through the element isolation insulating film, and having a multi-stage structure comprising an upper stage portion and a lower stage portion, the upper stage portion having a region surrounded by the element isolation insulating film in the semiconductor substrate, the lower stage portion having a diameter larger than that of the upper stage portion; and a contact plug connected to an end portion of the through plug on the frond surface side of the semiconductor substrate for connecting a conductive member formed above the front surface side of the semiconductor substrate to the through plug.