To obtain a high purity aluminum hydroxide by adding a supersaturated aluminate solution into a mixed suspension of a mother liquor having specific concentration of total sodium hydroxide and aluminum with a crystalline aluminum hydroxide to perform crystallization reaction.

The mother liquor having 50-700 g/l total sodium hydroxide concentration and 0-300 g/l aluminum concentration is fed to a crystallization reaction vessel and the temp. is setted to a prescribed temp. in a range of 10-200°C. Next the crystalline aluminum hydroxide is charged into the crystallization reaction vessel, controlled to have a prescribed concentration and kept as it is for 1-48 hr to dissolve a part of aluminum hydroxide and after that, the crystalline aluminum hydroxide is crystallized by mixing the supersaturated sodium aluminate solution intermittently, continuously and peridically with at least ≥3 hr, preferably ≥ 12 hr interval and setting to a crystallization temp. to perform the crystallization reaction and the liquid after the crystallization is used for an etchant for an etching process.