Abstract: For manufacturing a catheter having an inner tube forming a first lumen and an outer tube arranged coaxially with the inner tube, a second lumen being formed between the inner tube and the outer tube, a mandrel is inserted for retaining the first lumen into the inner tube, and an ultrasonic horn is applied to the outer surface of the outer tube for oscillating ultrasonic waves, thereby fusion bonding the inner surface of the outer tube to the outer surface of the inner tube. The particular method permits easily bonding the inner tube and the outer tube to each other. Also, since thermal deformation is small, it is possible to manufacture a catheter, in which the obstacle to the inflow of the inflation fluid into the inflatable member is very small, and which exhibits a rapid response to inflation and deflation of the inflatable member.