Coil structure used in endoscope and endoscope and treatment instrument including coil structure

A coil structure used in an endoscope, comprising:

• a coil tube disposed in or inserted into an insertion section of the endoscope, and formed by densely winding a wire rod having a substantially quadrilateral shape in cross section, both ends of the coil tube being fixed, the coil tube having a plurality of loops formed by winding the wire rod;
• wherein
• a first contact surface formed on one surface on a distal end side of each of the plurality of loops;
• a second contact surface formed on another surface on a proximal end side of each of the plurality of loops, the first contact surface coming into contact with the second contact surface,
• the first contact surface is formed as a convex curved surface arcuate in cross section, the convex curved surface having a radius of curvature with a center of curvature located within an internal lumen of the coil tube,
• the second contact surface is formed as a concave curved surface arcuate in cross section, the concave curved surface having a radius of curvature with a center of curvature located within an internal lumen of the coil tube, and
• the radius of curvature of the convex surface is larger than a radius of the coil tube and smaller than a radius of curvature of the insertion section of the endoscope when the insertion section is bent in a maximum state.