PURPOSE: To obtain a highly permeable coated composite membrane by passing a hollow fiber substrate through a polymeric soln. under constant tension at a constant speed to form a coating and subjecting the coated hollow fiber substrate to leaching in a leach liquor bath.

CONSTITUTION: The hollow fiber substrate 4 such as a polypropylene finely porous hollow fiber membrane is continuously unwinded from a delivery spool mechanism 2 and is wound-up on a receiving spool mechanism 22 under constant tension at a constant speed. Meanwhile, the substrate 4 is passed through inside of a storing tank 6 of the polymeric soln. 8 such as poly(trimethylsilylpropylene), is pulled through a coating die 10 in an axial direction to generate an axial direction annular propulsive stream and to form a uniform polymeric soln. coating on the substrate surface. The coated hollow fiber substrate 12 is taken out of the storing tank 6 and a part of the coating is evaporated to form a thin and dense layer. The remaining solvent is exuded from the coating layer by passing the substrate through an exudation liquid such as water in a passage 14 and then the substrate is air-dried and, wound-up on the spool mechanism 22.