In shape optimization, in which partial differential equations are defined and the shape with minimum cost is searched, an admissible family of shapes are given as being compact in some topology. Then minimizing of shapes for a cost implies an optimal shape, which we desire. The traction method is a power way to construct minimizing sequence of shapes for a cost, which guarantees the smoothness of shapes of the sequence. A reason of why the smoothness of shape is guaranteed is given from the point of view of mathematics.