A coil structure used in an endoscope includes a coil tube formed by densely winding a wire rod, both ends of the coil tube being fixed, a first contact surface formed on one surface of the wire rod on a distal end side, and a second contact surface formed on the other surface of the wire rod on a proximal end side, 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, a curvature center of which is located further on an inner side than an outer circumferential portion of the coil tube. For the convex curved surface, a curvature radius larger than a radius of the coil tube and smaller than a curvature radius during maximum bending of an insertion section is set.
