PURPOSE: To make endotoxin selectively adsorbed without markedly changing the composition of a solution to be treated by forming an endotoxin adsorbing filter characterized by which thickness is 10μm-5mm, water permeability is 1ml/min.m2.mmHg or more and void volume is 20% or more.

CONSTITUTION: A synthetic polymer prepared by using a monomer having an aromatic nitrogen-containing heterocyclic ring in its molecule as a constituent is held to the surface of a filter to form an endotoxin adsorbing filter characterized by which thickness is 10μm-5mm, water permeability is 1ml/min.m2.mmHg and void volume is 20% or more. As the base material constituting the filter, there are polyolefin such as polyethylene, polypropylene and halogenated polyolefin. As the aromatic nitrogen-containing heterocyclic ring, there are pyrrole, imidazole, thiazole, pyrazole, pyridine, pyrimidine and purine. The synthetic polymer prepared by using the monomer having the aromatic nitrogen- containing heterocyclic ring in its molecule as a constituent is introduced into the surface of the filter by a plasma graft polymerization method or a radioactive rays graft polymerization method.