To provide a catheter wherein an ionization radiation with minimum ratio is absorbed while it moves from a radiation source to a part of a vessel to be cured passing through an expanding balloon.

A catheter has a constitution wherein it has a slender shaft 1 with a base end part and an apex part 2 as a percutaneous and peroral access to a vessel part, an expandable balloon 8 arranged at the apex part 2 of the shaft 1, an expanding pipe cavity 7 extending by passing through the shaft 1 and opening in the balloon 8 and an ionization radiation source 12 which can be positioned in the balloon 8 and a gaseous expanding medium 9 filled for applying a pressure to the balloon.