Endoscope insertion unit, endoscope and endoscope system

An endoscope system comprising:

• an endoscope provided with an insertion unit,
• the insertion unit including:
• a first flexible portion arranged on a distal end of the endoscope insertion unit;
• and
• a second flexible portion coupled to a proximal end of the first flexible portion and having, over substantially an entire length thereof in a longitudinal direction, a constant outer diameter larger than a maximum outer diameter of the first flexible portion;
• and
• an insertion assisting device including:
• a roughly tube-shaped insertion tube having a low flexibility and an insertion hole formed from a proximal end to a distal end of the roughly tube-shaped insertion tube;
• and
• a roughly ring-shaped seal made of an elastic body, a proximal end of the seal being coupled to a distal end of the insertion tube, the proximal end of the seal having an outer diameter equal to the outer diameter of the insertion tube and having an inner diameter equal to the inner diameter of the insertion tube so as to be smoothly continuous with the distal end of the insertion tube, the seal including on a distal end thereof an opening whose inner diameter is equal to or slightly smaller than the outer diameter of the second flexible portion of the insertion unit of the endoscope, the opening being configured so as to provide sealing with respect to an outer periphery of the second flexible portion.