This invention relates to a catheter equipped detachably at the tip thereof with a cylindrical expansion holding member of expanding the inner diameter of the lumen of the blood vessel or other tubular organ and for holding it in an expanded state either temporarily or permanently. The tip of the catheter which supports this member has a sufficiently smaller outer diameter than those of the other portions and the outer diameter of the expansion holding member is equal to, or smaller than, the outer diameters of the rest of portions of the catheter even when the expansion holding member is supported at the tip. The expansion holding member is made of a shape memory alloy so that it can expand or contract in the radial direction with a temperature change. Expansion or contraction of this member is made by causing the outflow of a heated or cooled liquid from a side hole bored at said tip of the catheter and bringing it into contact with the expansion holding member.
