An adapter and its connection structure having a plurality of tubes bundled together as one that can be connected to be made shorter in the diameter direction to transport fluid to and from artificial organs and connect electric wires to the artificial organs without changing design of the artificial organs. The adapter and connecting structure connect multi-lumen and single tubes so their passages communicate with each other. The first attachment member, attached to the multi-lumen tube, includes first ports having an outer diameter substantially the same a lumen opening diameter at the end portion of the multi-lumen tube to fit into the lumen openings, and a first cutout corresponding to one of the lumen openings. The second attachment member, attached to the single tube, includes second ports that fit into the lumen of the single tube. The number of the first and second ports is the same.
