A dialyzer in which a nearly cylindrical case is provided therein with a hollow fiber bundle made of a plurality of hollow fiber membranes, a blood flow path formed by lumens of the hollow fiber membranes, a dialyzate flow path formed by a space between the inner wall of the case and the hollow fiber membranes and a cylindrical heat-shrinkable tube provided on the outside of the hollow fiber bundle and at least a part of which can be shrunk by heating, and a method for manufacturing the dialyzer.