An introducer having a dilator positioned in a sheath is inserted into a first blood vessel and, thereafter, only the dilator tube is drawn out of the first blood vessel and the sheath while the sheath remains in the first blood vessel. A diagnostic instrument having an outer diameter smaller than a maximum outer diameter permitting insertion into the sheath tube is inserted into the second blood vessel, and a stenosis of the second blood vessel is diagnosed. While keeping the sheath indwelling in the first blood vessel, a therapeutic instrument or a catheter permitting the therapeutic instrument to be inserted therein, which therapeutic instrument or catheter has a maximum outer diameter permitting insertion into the sheath tube, is inserted into the second blood vessel, and the second blood vessel is treated.