A joint to be attached to a port of a micro fluidic element, has an outer peripheral surface of truncated subulate shape at a front end portion of the joint to form a front surface as a top of the truncated subulate shape, and a cuneiform space in an inside of the joint at the front end portion of the joint, the joint is made of an elastomer, and the cuneiform space includes a front end overlapping the front surface as seen in a direction along which a tube is guided by the cuneiform space when the tube is inserted into the joint so that the tube is enabled to penetrate the front surface through the front end.