A balloon catheter ablation system having a balloon (6) at a distal end of a catheter shaft (1). An electrode (11) for delivery of radiofrequency current and a temperature sensor (12) are arranged inside the balloon (6), Bipolar electrode pair (16a, 16b) are arranged on the outside of the balloon (6), with the balloon (6) sandwiched therebetween. When a fluid is injected into the inside of the balloon (6) by using a syringe (24), the balloon (6) expands and comes in close contact with a target site (S) of tissue. A vibration generator (25) agitates the fluid inside the balloon (6) by using vibration. A radiofrequency generator (31) conducts electricity between the electrode (11) for delivery of radiofrequency current and a counter electrode (13) and heats the fluid inside the balloon (6). Accordingly, the balloon (6) ablates the target site (S). The impedance between the bipolar electrode pair (16a, 16b) decreases as the ablation progresses. Accordingly, the progress of the ablation can be ascertained from the measurement results of an electrical impedance measuring potential amplifier (41).
