Thermal conduction to a non-target tissue is unfailingly blocked off to select and effectively ablate only a target tissue. A catheter shaft 1 includes an outer, intermediary and inner tubes 2, 3, 4 which are slidable to each other. An outer balloon 5 is arranged between distal ends 2A, 4A of the outer and inner tubes 4, 5, while an inner balloon 6 is arranged between distal ends 3A, 4A of the intermediary and inner tube 4. The outer balloon 5 is filled with a low thermal conductive gas. Filling solution filled inside the inner balloon 6 is subjected to radiofrequency heating by supplying electricity to an electrode. The outer balloon 5 is held inside a luminal organ and the thermal conduction from the inner balloon 6 is blocked off, thus the thermal energy of the inner balloon 6 selectively ablates a target tissue via a close-contact portion 14.
