PURPOSE: To chemically modify a cellulosic dialysis membrane by treating the cellulosic dialysis membrane such as flat film, tube film with a polar solvent dissolved acid chloride, acid anhydride, acid and ester, etc.

CONSTITUTION: The acid chloride, acid anhydride, acid, ester and ketene, etc., are dissolved in the polar solvent, and a catalyst and an auxiliary, etc., if desired, are added to prepare the treating solution. This treating solution is filtered along a surface of the cellulosic dialysis membrane to chemically modify the cellolosic dialysis membrane. As the device for this purpose, e.g. a pipe system 14 is a looped pipe and storage containers 1, 2, 3, 4, 6 are connected to the pipe in front and in the rear via metering feed devices 17, and the metering feed devices 17 enable to feed a reactant, a solvent, and a detergent to compartments of membrane modules 13 for a specified time and with a specified quantity via the looped pipe.