A coil structure used in an endoscope 1 includes a coil tube 33, both ends of which are fixed, a wire rod 40 densely wound to form the coil tube 33, a first contact surface 41 formed on one surface of the metal wire 40 on a distal end side, and a second contact surface 42 formed on the other surface of the metal wire 40 on a proximal end side, the first contact surface 41 coming into contact with the second contact surface 42. The first contact surface 41 is formed as a convex curved surface arcuate in cross section, a curvature center O1 of which is located further on an inner side than an outer circumferential portion of the coil tube 33. For the convex curved surface, a curvature radius R1 larger than a radius D/2 of the coil tube 33 and smaller than a curvature radius SR during maximum bending of an insertion section 2 is set.
