One of the problems in the practical use of 3D printers using cement-based materials is how to withstand the tensile force. In general, cement-based materials can withstand compressive forces. Therefore, reinforced concrete structures are applied as composite structures with steel materials such as reinforcing bars that can withstand tensile force. In this study, we developed a composite structure, in which the outer part was laminated with mortar exclusively for 3D printers, and the inner part was filled with ultra-high strength fiber-reinforced concrete as a substitute of reinforcing bars. When the test piece was manufactured and its mechanical properties were tested by experiments, it was concluded that the desired strength had been obtained. Then, a large shell-shaped bench, whose outer dimensions comprised a width of 7 m, depth of 5 m, and height of 2.5 m, was manufactured. The design used topology optimization that derived a shape with high structural rationality; the total weight was reduced by approximately 60%. The shape derived by topology optimization would have been difficult to construct with a formwork, and we were able to exploit the advantages of using a 3D printer.