Recently, it has become possible to easily design and fabricate robotic systems in the laboratory and at home due to the recent development of 3D printer technology. On the other hand, the strength of the plastic materials used in reasonably priced 3D printers and the accuracy of the printed parts are generally low. These problems affect the part-joining quality. Therefore, this paper describes a design method inspired by ancient Japanese wooden joinery techniques for assembling 3D-printed parts and presents the design of a robotic hand as its application. The joinery techniques use special shapes to assemble components and allow us to assemble the robotic hand without glue, screws or nails and to easily disassemble it.