A flexible tube formed by interlocking a first edge of a single strip material with a second edge of the strip material that adjoins the first edge by spirally winding the strip material, allowing passage of a solid object, and having a specific length, wherein the flexible tube has a shape of an ellipse in cross-section perpendicular to a direction of the passage, the ellipse having a shape that prevents a torsional force caused by torsion from propagating to a part on which a torsional force concentrates in a flexible tube with a circular cross-section perpendicular to the direction of the passage, and wherein the ellipse has a shape that creates a space for a cable between an outer surface of the ellipse and an inner surface of a covering material having a circular cross-section with a diameter greater than a length of a major axis of the ellipse.