Flexible tube, and insertion device and endoscope employing the flexible tube

A flexible tube comprising:

• a first spiral tube including:
• a first densely wound portion in which a first element wire is wound such that adjacent turns are in tight contact with each other with an initial tension, and
• a first loosely wound portion in which the first element wire is wound such that adjacent turns are spaced from each other by a constant distance, the first densely wound portion and the first loosely wound portion alternating in a direction of a central axis of the first spiral tube, and
• a second spiral tube including:
• a second densely wound portion in which a second element wire is wound such that adjacent turns are in tight contact with each other with an initial tension, and
• a second loosely wound portion in which the second element wire is wound such that adjacent turns are spaced from each other by a constant distance, the second densely wound portion and the second loosely wound portion alternating in a direction of a central axis of the second spiral tube,
• wherein:
• the first spiral tube and the second spiral tube have an equal length in a longitudinal direction and are overlaid on each other,
• the first spiral tube is fitted in and fixed at both ends to the second spiral tube to have a positional relationship that makes:
• the first loosely wound portion of the first spiral tube opposed to the second densely wound portion of the second spiral tube,
• the first densely wound portion of the first spiral tube opposed to the second loosely wound portion of the second spiral tube, and
• the central axis of the first spiral tube align with the central axis of the second spiral tube.