A coupling assembly comprising: first and second plate structures (11, 12) placed on each other; and a coupler coupling the first and second plate structures, wherein the first plate structure includes a first insertion hole and a first serration located on an outer surface of the first plate structure and surrounding the first insertion hole, the outer surface facing away from the second plate structure, wherein the second plate structure includes a second insertion hole differing in inner diameter from the first insertion hole, and wherein the coupler includes a contact structure including a second serration and a third insertion hole, the second serration contacting the outer surface of the first plate structure and meshing with the first serration of the first plate structure, a shaft structure inserted into the first, second, and third insertion holes, the shaft structure including a shaft portion with a fixed engagement portion fixedly engaged with an outer surface of one of the second plate structure and the contact structure, and a pressing structure engaged with the shaft structure and pressing an outer surface of the other of the second plate structure and the contact structure, wherein the shaft structure has an outer diameter equal to inner diameters of the second insertion hole of the second plate structure and the third insertion hole of the contact structure, and the inner diameters of the second and third insertion holes are smaller than an inner diameter of the first insertion hole, characterized in that the outline of the radial cross-section of the shaft portion is circular and has the same shape as the contours of the first and second insertion holes (11a, 12a). first and second plate structures (11, 12) placed on each other; and a coupler coupling the first and second plate structures, wherein the first plate structure includes a first insertion hole and a first serration located on an outer surface of the first plate structure and surrounding the first insertion hole, the outer surface facing away from the second plate structure, wherein the second plate structure includes a second insertion hole differing in inner diameter from the first insertion hole, and wherein the coupler includes a contact structure including a second serration and a third insertion hole, the second serration contacting the outer surface of the first plate structure and meshing with the first serration of the first plate structure, a shaft structure inserted into the first, second, and third insertion holes, the shaft structure including a shaft portion with a fixed engagement portion fixedly engaged with an outer surface of one of the second plate structure and the contact structure, and a pressing structure engaged with the shaft structure and pressing an outer surface of the other of the second plate structure and the contact structure, a contact structure including a second serration and a third insertion hole, the second serration contacting the outer surface of the first plate structure and meshing with the first serration of the first plate structure, a shaft structure inserted into the first, second, and third insertion holes, the shaft structure including a shaft portion with a fixed engagement portion fixedly engaged with an outer surface of one of the second plate structure and the contact structure, and a pressing structure engaged with the shaft structure and pressing an outer surface of the other of the second plate structure and the contact structure, wherein the shaft structure has an outer diameter equal to inner diameters of the second insertion hole of the second plate structure and the third insertion hole of the contact structure, and the inner diameters of the second and third insertion holes are smaller than an inner diameter of the first insertion hole, characterized in that the outline of the radial cross-section of the shaft portion is circular and has the same shape as the contours of the first and second insertion holes (11a, 12a).