PURPOSE: To obtain the titled hydroxyalkyl ether, by reacting galactomannan with an alkylene oxide in the presence of an alkali metal hydroxide by using a medium consisting of a hydrophilic organic solvent and water.

CONSTITUTION: Galactomannan is reacted with an alkylene oxide in the presence of at least 0.4mol, per anhydrosugar unit of galactomannan, of an alkali metal hydroxide, by using a medium consisting of a hydrophilic organic solvent and water. Said solvent refers to an organic solvent of such hydrophilicity that it will not separate from water when it contains at least 30wt% water, and preferable examples of the solvents include 2-propanol, acetone, and methyl ethyl ketone. The ratio of the hydrophilic solvent to water is preferably in the range of 30:70W95:5. Galactomannan which can be used includes a galactomannan- based substance such as guar gum or locustbean gum, and raw materials for these gums, such as guar beans or locust beans.