A polymer compound derivative, obtained by modifying part of the hydroxy or amino groups of a polymer compound having the hydroxy or amino groups with molecules of a compound represented by the following general formula (I): A-X--Si(Y).sub.nR.sub.3-n (I), where A represents a reactive group which reacts with a hydroxy or amino group, X represents an alkylene group which has 1 to 18 carbon atoms and which may have a branch, or an arylene group which may have a substituent, Y represents a reactive group which reacts with a silanol group to form a siloxane bond, R represents an alkyl group which has 1 to 18 carbon atoms and which may have a branch, or an aryl group which may have a substituent, and n represents an integer of 1 to 3.