Abstract: A device for occluding a fluid passageway such as a blood vessel, particularly a human blood vessel, which is of the balloon type, adapted to be inserted, by means of a catheter structure, into a blood vessel and moved thereby to the desired point of occlusion, at which the ballon is expanded by means of a fluid conducted through the catheter, thereby firmly securing the occlusion device to the vessel side wall. Novel means are provided, including a dual valve mechanism for insuring the retention of the expanding fluid in the device, thereby providing efficient sealing of the balloon to substantially eliminate the possibility of fluid leakage upon separation of the occluding device from the inserting catheter structure.