A conductive material mixed composition obtained by synthesizing a piperidyl group-containing high-molecular-weight polymer or copolymer that is composed of the constitutional unit represented by general formula (1) below and insoluble in tetrahydrofuran (THF) by polymerization in a liquid phase, immediately followed by mixing a conductive material with the polymer or copolymer in the liquid phase: ##STR00001## wherein R.sup.1 represents a hydrogen atom, a hydroxyl group, an oxyl radical, an alkyl group having 1 to 18 carbon atoms, or an alkoxy group having 1 to 18 carbon atoms; R.sup.2 and R.sup.3 each independently represent a hydrogen atom or a methyl group; R.sup.4 represents a hydrogen atom, an alkali metal atom, an alkyl group having 1 to 50 carbon atoms, an alkenyl group having 1 to 50 carbon atoms, an alkyl group having 1 to 50 carbon atoms, or a haloalkyl group having 1 to 50 carbon atoms; n represents a number of not less than 30; and m represents 0 or a positive number.