In this report, the new method of topology optimization technique that is able to deal with the three-dimensional structure with constant cross section is described. In actual design field, the structure with constant cross section is widely adopted because of manufacturing restrictions and efficient design and maintenance. A key technology is to form a longitudinal group by collecting finite elements along the sweeping curve, and the structure is constructed with these groups. Since the design variables are allocated to each group instead of to each finite element, the total number of design variables is drastically reduced. Illustrative examples are shown and the effectiveness of the proposed method is discussed.