A dialyzer for blood treatment including a semipermeable membrane which is made of a hydrophobic polymer and a hydrophilic polymer, and has a water permeating performance drying of or higher relative to that before drying. The dialyzer has a vitamin B12 clearance not smaller than 135 ml/mm per 1.6 m² or the amount of the hydrophilic polymer eluted from the semipermeable membrane is not higher than 10 ppm. A dialyzer for blood treatment is light-weight, easy to handle, and exhibits a reduced elution of the hydrophilic polymer procedures for producing a dialyzer containing the semipermeable membrane and a process for producing a hollow fiber membrane for use in blood treatment are described.