PURPOSE: To provide the subject deriv. comprising dialkylaminoalkyl ether- hydroxyalkyl ether-long chain aliphatic chain ether of polygalactomannan and used for a thickening agent of an aq. fluid capable of being regulated in its viscosity by the addition of a reagent.

CONSTITUTION: A hydrophilic group reactant such as an alkylene oxide (e.g. ethylene oxide) is reacted with polygalactomannan in the presence of an alkali catalyst to introduce a 2-4C hydroxyalkyl ether group and a hydrophobic group reactant such as long chain aliphatic chain epoxide (e.g. stearylglycidyl ether) is reacted with the reaction product in a solvent such as 2-propanol to derive a 8-28C long chain aliphatic chain ether group and, finally, a dialkylaminoalkyl etherifying agent having a 1-6C alkyl group and having 12 or less carbon atoms in total (e.g. 3-dimethylamino-1,2-epoxypropane, etc.) is reacted with the obtained product to obtain a dialkylaminoalkyl ether-hydroxyalkyl ether-long chain aliphatic chain ether deriv. of polygalactomannan.