A dilator includes a hollow shaft having an outer diameter that is smaller at a distal end than at a proximal end, and a connector connected to the proximal end of the shaft. A spirally-arranged protruding portion protruding outwardly is provided on an outer peripheral surface of the shaft. The spirally-arranged protruding portion has gaps between adjacent portions of the spirally-arranged protruding portion along a longitudinal axis of the shaft. The shaft has a first position and a second position along the longitudinal axis, the second position being located distal to the first position. The outer diameter of the shaft is smaller at the second position than at the first position. An inclination angle of the spirally-arranged protruding portion relative to the longitudinal axis is different at the second position than at the first position.
