A spiral unit includes: a tube body having a spiral fin arranged on an outer peripheral surface thereof； a tubular taper section which is provided along the longitudinal axis, has a proximal part close to the tube body and a distal part apart from the tube body, has an outer periphery that is diameter-reduced toward the longitudinal axis more at the distal part than at the proximal part along the longitudinal axis, and exerts a diameter reducing force to an outer peripheral surface of the insertion section at the distal part in the range where sliding is possible in a periaxial direction of the longitudinal axis； and a reduced diameter regulator which is removably arranged in the taper section, holds a state of expanding an inner diameter of the taper section so that insertion of the insertion section into the taper section is allowed, to regulate reduction of the inner diameter of the taper section, and is removed from the taper section along with the insertion of the insertion section into the taper section, thereby exerting the diameter reducing force to the taper section more at the distal part than at the proximal part on the outer peripheral surface of the insertion section in the range where sliding is possible in the periaxial direction of the longitudinal axis.
