PURPOSE: To improve the efficiency in treating a body fluid by using the titled permselective hollow yarn provided with a fin, having a deformed cross section and having a crimp.

CONSTITUTION: For example, a plasticizer such as polyethylene glycol is added to cellulose diacetate, a spinneret having a notched part on the outer peripheral part of the double annular part is used and the material is melt-spinned while blowing gaseous nitrogen into the central part. The obtained finned hollow yarn is supplied onto a wire mesh, hot water is sprayed to extract the plasticizer in the hollow yarn, the yarn is then cooled and a finned and crimped hollow yarn is obtained. A blood dialyzer is formed by using the bundle of hollow yarns.