To provide a porous membrane reduced in or inhibited from the elution of an endotoxin adsorbent from a porous surface, being highly safe, and stably having an excellent endotoxin adsorptivity and to provide a simple method for producing the porous membrane.

The porous membrane is the one coated with a quaternary ammonium salt in which the four aliphatic alkyl groups are independent of each other, the total of the numbers of carbon atoms of the aliphatic alkyl groups is 22-40, and at least two of the aliphatic alkyl chains have each 10 carbon atoms. The method for producing the porous membrane comprises dissolving the quaternary ammonium salt in which the four aliphatic alkyl groups are independent of each other, the total of the numbers of carbon atoms of the aliphatic alkyl groups is 22-40, and at least two of the aliphatic alkyl chains have each 10 carbon atoms in an organic solvent and bringing a porous membrane into contact with the solution to coat at least a part of the surface of the porous membrane with the ammonium salt.