Disclosed herein is a method for producing potassium fluorotantalate crystals, comprising heating, to 60° C. or higher, a starting solution prepared by adding hydrofluoric acid and hydrochloric acid, and, if necessary, water to a highly pure tantalum solution, adding potassium chloride to this starting solution, and cooling the mixture to 40° C. or lower at a cooling rate of less than 15° C./h to precipitate potassium fluorotantalate crystals, wherein the concentration of the hydrofluoric acid in the starting solution is not more than 20% by weight. By this method, highly pure, large-sized potassium fluorotantalate crystals can successfully be obtained in high yield. Moreover, disadvantages to equipment and operation in the prior art can be overcome to a significant extent with this method.