The spiral tube constituting the flexible tube used in the insertion portion is constructed by winding a strip of strand of wire in a spiral shape and covering the outer layer from its outer periphery. In the strands, a plurality of adjustment portions formed by holes or bottomed holes are arranged along the longitudinal direction of the stripes in such a manner that the sustaining portions are arranged at intervals of a half circumference of the spiral. By the winding of the spiral, adjacent portions of the strands of wire are shifted in the circumferential direction of the longitudinal axis so as not to be continuous in the direction parallel to the longitudinal axis.挿入部に用いられる可撓管を構成する螺旋管は、帯状の素線を密巻で螺旋状に巻回し、その外周側から外層部に被覆されて構成される。素線は、帯状における長手方向に沿って、螺旋の半円周以上の間隔で維持部を配置して、孔又は有底孔で形成される複数の調整部が連設される。螺旋の巻回により隣り合った素線に、それぞれに形成される維持部同士が長手軸に平行な方向に連続しないよう長手軸の周方向にずれて配置される。
