To provide balloon adaptors which can improve the operability thereof by enabling the dilation of blood vessels with a single balloon catheter only.

The balloon adaptors (1, 11, 21 and 31) are made into a shape that allows the shielding of the outer circumference of a balloon of the catheter used in blood vessels and formed out of an elastic material expandable or shrinkable to match the expansion or shrinking of the balloon. In the balloon catheters (51 and 51A), the balloon adaptors (1, 11, 21, 31 and 41) are mounted on the outer circumference of the balloon in the balloon catheter used in blood vessels.