Provided is a driving force transmission structure capable of driving a conventional winding drum while connecting a short transmission shaft while securing rigidity against torsion by a columnar or cylindrical transmission shaft. A circular transmission shaft includes a circular shaft portion having a circular cross section, and a rectangular shaft portion having a rectangular cross section provided continuously at the tip of the circular shaft portion. The rectangular shaft portion 8 is configured to have a size surrounding the first rectangular shaft constituting member 80a and a first rectangular shaft constituting member 80a having a shape obtained by partially cutting one side of the cylindrical member having a rectangular cross section. And a second rectangular shaft constituting member 80b having a shape obtained by partially cutting one side of the cylindrical member having a rectangular cross section, and each side of the second rectangular shaft constituting member 80b. Of these sides, the cut side is disposed on the opposite side of the cut side of the first rectangular shaft constituting member 80a. [Selection] Figure 4【課題】円柱形または円筒形の伝達軸により捻れに対する剛性を担保しつつ、短尺な伝達軸を連結させながら従来型の巻回ドラムを駆動させることができる駆動力伝達構造を提供する。【解決手段】円形伝達軸は、断面を円形としてなる円形軸部７と、この円形軸部７の先端に連続して設けられる断面矩形の矩形軸部８とを備える。矩形軸部８は、断面矩形の筒状部材のうち一辺を部分的に切断された形状とした第１の矩形軸構成部材８０ａと、第１の矩形軸構成部材８０ａを包囲する大きさに構成されるとともに、断面矩形の筒状部材のうち一辺を部分的に切断された形状とした第２の矩形軸構成部材８０ｂとで構成されており、第２の矩形軸構成部材８０ｂの各辺のうち切断された辺は、第１の矩形軸構成部材８０ａの各辺のうち切断された辺の反対側に配置されている。【選択図】図４
