The present invention relates to graft polymer which is characterized in that it is formed by graft polymerization of structural units containing. a quaternary ammonium group represented by general formula (A), ##STR1## (R.sub.2 and R.sub.3 each represent an alkyl group with from 1 to 3 carbons, and R.sub.4 represents an alkyl group with from 3 to 18 carbons. X represents at least one type of ion selected from halogen, sulphate, hydroxide and carboxylic acid ions.), preferably to graft polymer where the structural units containing a quaternary ammonium group represented by general formula (A) are structural units which are represented by general formula (I), ##STR2## (R.sub.1 represents at least one species selected from hydrogen, the methyl group and the ethyl group, and n represents an integer in the range 1 to 12. A represents at least one species selected from O, S and NR.sub.5. R.sub.5 represents hydrogen or an alkyl group with 1 to 12 carbons.) or graft polymer formed by the graft polymerization of structural units containing a quaternary ammonium group and structural units containing an alkoxypolyalkylene glycol moiety represented by the general formula (B), --(R.sub.5 O).sub.n R.sub.6 (B) (n represents an integer in the range 1 to 100. R.sub.5 represents a straight-chain or branched alkylene group with from 1 to 4 carbons, and R.sub.6 represents at least one species selected from hydrogen and straight-chain or branched alkyl groups with from 1 to 4 carbons.). By applying or incorporating the graft polymer to/in medical devices which are left in the body for a long period, it is possible to prevent microbial infection arising via these medical devices.