An alginic acid derivative comprising a group represented by formula (I) below, excluding a part to the right of the broken line in the formula, at some of the carboxyl groups of at least one selected from the group consisting of alginic acid, esters thereof, and salts thereof, wherein the some of the carboxyl groups form amide bonds with a terminal amino group of the group represented by formula (I): wherein -A 1 - is a linker selected from the group consisting of the following formulae, excluding parts outside the broken lines at both ends of each formula: in which each R 1 independently represents a group selected from the group consisting of a hydrogen atom, methyl group, isopropyl group, isobutyl group, sec-butyl group, hydroxymethyl group, 2-hydroxyethyl group, thiolmethyl group, methylthioethyl group, carboxymethyl group, carboxyethyl group, aminocarbonylmethyl group, aminocarbonylethyl group, aminobutyl group, guanidinopropyl group, benzyl group, 4-hydroxybenzyl group, 3-indolylmethyl group and 4-imidazoylmethyl group and a propane-1,3-diyl group that forms a ring together with a carbon atom to which the R 1 binds and a nitrogen atom to which that carbon atom binds; each R 2 is independently a group selected from the group consisting of a hydrogen atom, methyl group, isopropyl group, isobutyl group, sec-butyl group, hydroxymethyl group, 2-hydroxyethyl group, thiolmethyl group, methylthioethyl group, carboxymethyl group, carboxyethyl group, aminocarbonylmethyl group, aminocarbonylethyl group, aminobutyl group, guanidinopropyl group, benzyl group, 4-hydroxybenzyl group, 3-indolylmethyl group, and 4-imidazoylmethyl group, and a propane-1,3-diyl group that forms a ring together with a carbon atom to which the R 2 binds and a nitrogen atom to which that carbon atom binds; n is an integer from 1 to 18; m is an integer from 1 to 9; and j is an integer from 0 to 9, provided that -CH 2 CH 2 - as -A 1 - is excluded. in which each R 1 independently represents a group selected from the group consisting of a hydrogen atom, methyl group, isopropyl group, isobutyl group, sec-butyl group, hydroxymethyl group, 2-hydroxyethyl group, thiolmethyl group, methylthioethyl group, carboxymethyl group, carboxyethyl group, aminocarbonylmethyl group, aminocarbonylethyl group, aminobutyl group, guanidinopropyl group, benzyl group, 4-hydroxybenzyl group, 3-indolylmethyl group and 4-imidazoylmethyl group and a propane-1,3-diyl group that forms a ring together with a carbon atom to which the R 1 binds and a nitrogen atom to which that carbon atom binds; each R 2 is independently a group selected from the group consisting of a hydrogen atom, methyl group, isopropyl group, isobutyl group, sec-butyl group, hydroxymethyl group, 2-hydroxyethyl group, thiolmethyl group, methylthioethyl group, carboxymethyl group, carboxyethyl group, aminocarbonylmethyl group, aminocarbonylethyl group, aminobutyl group, guanidinopropyl group, benzyl group, 4-hydroxybenzyl group, 3-indolylmethyl group, and 4-imidazoylmethyl group, and a propane-1,3-diyl group that forms a ring together with a carbon atom to which the R 2 binds and a nitrogen atom to which that carbon atom binds; n is an integer from 1 to 18; m is an integer from 1 to 9; and j is an integer from 0 to 9, provided that -CH 2 CH 2 - as -A 1 - is excluded. <EOS>