PURPOSE: To produce the grains having angular form and stabilized crystal structure by adding an aq. solution contg. oxalate ions, ammonium ions and hydrogen ions to a rare earth element salt aq. solution in an amount enough to provide specified molar ratios and concn. of the components in the resultant mixed solution and aging the mixed solution.

CONSTITUTION: In this production, ammonium oxalate alone or at least two compounds selected from ammonium oxalate, oxalic acid and ammonia are dissolved in water to obtain a solution used as the first solution, and also, an aq. solution of a water soluble salt of a rare earth element is used as the second solution. At this time, if necessary, an acid other than oxalic acid is added to one or both of the first and second solutions to adjust the component contents in the both solutions so as to meet the following inequalities (1) to (4). Then, the first and second solutions are mixed together to form a precipitate and the resultant solution contg. the precipitate is heated to 30 to 100°C to change the form of all the precipitate grains into angular form, and also, after confirming that the crystal structure of the precipitate is completely changed into its stable structure, the resultant precipitate is separated by filtration and washed with water to produce the objective angular grains of a rare earth element ammonium oxalate double salt.