Abstract: A balloon catheter has an inner tube communicating with a blood introduction port provided at a tip of the balloon portion. The inner tube is affixed to the inner wall of the catheter tube by adhesion, melt-bonding, or integral formation. The balloon catheter has a small channel resistance in the channel where the pressurized fluid which expands and contracts the balloon portion flows, even when the catheter tube has been bent, and enables expansion and contraction of the balloon portion with a good response. Another balloon catheter has the balloon portions at a range of the end sides of the balloon portions smaller in sectional area compared with the balloon portions at the tip sides. The catheter is able to effectively prevent side effects which occur only rarely but which are serious when they do occur. The catheter impairs as little as possible the effect of assisting the heart action inherent to the IABP method without special measurement of the diameter of the patient's blood vessels.