An insertion device includes a base member formed into integral one from a single member, a distal side ring member maintaining liquid-tightness between a distal side circular surface of the base member and an inner peripheral portion of a rotating tubular member, and a proximal side ring member maintaining liquid-tightness between a proximal side circular surface of the base member and the inner peripheral portion of the rotating tubular member. The insertion device includes a movement range regulating portion regulating a movement range of the rotating tubular member in directions parallel to the longitudinal axis so that the distal side ring member and the proximal side ring member come into contact with the inner peripheral portion of the rotating tubular member, and an axial center position regulating portion regulating a position of an axial center of the rotating tubular member so that the rotating tubular member is coaxial with the longitudinal axis.
