This invention provides a viscosity modifier for high concentration dispersion of inorganic fine particles which can reduce, by giving excellent viscosity modifying function to a high concentration dispersion of inorganic fine particles (e.g., printing ink composition, functional paste composition or paint composition, of high concentration dispersion of inorganic fine particles), the occurrence of inconvenience in a printing step or coating step for the manufacture of components or substrates, and also provides a high concentration dispersion of inorganic fine particles which contains said viscosity modifier. This viscosity modifier comprises, as a main component, a reaction product having 2 to 20 phosphate groups or carboxyl groups in molecular structure, which reaction product is produced from a reaction between a polyol having two to six hydroxyl groups and a phosphate group-introducing material or a carboxyl group-introducing material at a molar ratio of hydroxyl group to phosphate group-introducing material or carboxyl group-introducing material of 3:1 to 1:1.