Medical catheter apparatus used for implanting a vascular stent into a vessel of a living body comprising a catheter including a distal portion where a balloon attaching a vascular stent is provided, and a sheath into which the catheter is inserted and to be moved relative to the catheter between the section where it covers the balloon on which a vascular stent is mounted and the section where it exposes the vascular stent mounted on the balloon. The sheath comprises a first tubular member covering the distal portion at which the balloon is provided, and a second tubular member covering the proximal portion of the catheter and connected with the first tubular member. The first tubular member is formed of a tubular member capable of flexible deformation and the second tubular member is formed of a tubular member less extensible in the axial direction than the first tubular member.