To obtain a derivative which gives a highly transparent aqueous solution and has an excellent viscosity improving effect with a small amount of addition by substituting a part of or all of hydrogen atoms in hydroxy groups of a polysaccharide or its derivative with a specific hydrophobic or hydrophilic group.

A novel polysaccharide derivative is obtained by reacting a polysaccharide or its derivative such as celluloses, guar gums, starches and the like with compounds of formulae III and IV (wherein D stands for a halogen, a hydroxy group, a 3-6C epoxidated alkyl group or a halogen-substituted 1-6C alkyl group; A stands for an oxygen or carbonyl group; R stands for a 4-43C alkyl group; n is 0 or 1; and X is a halogen) to substitute 0.0001-1.0, per a unit monosaccharide residual group on the average, of hydrogen atoms of hydroxy groups with a substituent group formula I (wherein B stands for a single bond of 1-6C alkylene group) and 0.01-2.0, per a unit monosaccharide residual group on the average, of hydrogen atoms of hydroxy groups with a substituent group of formula II (wherein M1 and M2 each stand for H or a monovalent cation). This novel polysaccharide derivative gives an emulsion with an excellent stability exhibiting a small viscosity change over the presence of salts the temperature.