In a semiconductor integrated circuit device and a method of designing the same, design information about circuit cells each having a desired function are described as objects according to selected purposes. The pieces of design information are registered in a cell library as cell information capable of forming any of substrate potential fixed cells and substrate potential variable cells. Further, a data sheet common to the substrate potential fixed cell and the substrate potential variable cell is offered to a user, so that the user is able to make a selection according to the user's purposes. The substrate potential fixed cells and the substrate potential variable cells are mixed together on a semiconductor chip so as to be properly used according to the functions or the like of circuit portions in which the cells are used.