A medical catheter apparatus comprising: a catheter including an outer periphery on which a cylindrical vascular stent is to be mounted and a distal portion attaching a balloon which is to be inflated with supply of an expansion medium to expand the stent in diameter, being provided with at least an expansion medium supplying channel extending from the distal portion to a proximal portion for supplying the expansion medium to expand the balloon, and a guide wire insertion channel extending from the distal portion to a middle portion for inserting a guide wire, and a sheath into which the catheter is inserted, covering the outer peripheral of the catheter from its distal side where the balloon attaching the vascular stent is positioned through its proximal side, and moving relative to the catheter between the section where it is covering the balloon attaching the vascular stent and the section where it is supposed to expose the vascular stent attached to the balloon, wherein the catheter is provided at the middle portion with a guide wire leading-out opening for leading out the guide wire inserted from the distal portion of the catheter toward the side direction of the catheter, the sheath is provided at a middle portion with a guide wire drawing-out opening for drawing the guide wire drawn from the catheter out of the sheath, a guide wire drawing-out guiding mechanism is provided between the guide wire leading-out opening and the guide wire drawing-out opening for connecting the guide wire leading-out opening and the guide wire drawing-out opening, and for guiding the guide wire led out from the guide wire leading-out opening to the guide wire drawing-out opening, and the guide wire drawing-out guiding mechanism has a variable length from the guide wire leading-out opening to the guide wire drawing-out opening and is configured to vary the length with the relative movement of the sheath with respect to the catheter. a catheter including an outer periphery on which a cylindrical vascular stent is to be mounted and a distal portion attaching a balloon which is to be inflated with supply of an expansion medium to expand the stent in diameter, being provided with at least an expansion medium supplying channel extending from the distal portion to a proximal portion for supplying the expansion medium to expand the balloon, and a guide wire insertion channel extending from the distal portion to a middle portion for inserting a guide wire, and a sheath into which the catheter is inserted, covering the outer peripheral of the catheter from its distal side where the balloon attaching the vascular stent is positioned through its proximal side, and moving relative to the catheter between the section where it is covering the balloon attaching the vascular stent and the section where it is supposed to expose the vascular stent attached to the balloon, wherein the catheter is provided at the middle portion with a guide wire leading-out opening for leading out the guide wire inserted from the distal portion of the catheter toward the side direction of the catheter, the sheath is provided at a middle portion with a guide wire drawing-out opening for drawing the guide wire drawn from the catheter out of the sheath, a guide wire drawing-out guiding mechanism is provided between the guide wire leading-out opening and the guide wire drawing-out opening for connecting the guide wire leading-out opening and the guide wire drawing-out opening, and for guiding the guide wire led out from the guide wire leading-out opening to the guide wire drawing-out opening, and the guide wire drawing-out guiding mechanism has a variable length from the guide wire leading-out opening to the guide wire drawing-out opening and is configured to vary the length with the relative movement of the sheath with respect to the catheter.