PURPOSE: To simplify an analysis operation by shortening a reagent preparing time and to enable settling of a problem on a working environment, by using Dotite-alfusson as a reagent.

CONSTITUTION: Alizarincomplexon and lanthanum are chelated in the ratio 1:1, 1g Dotite-alfusson in a violet powder and on a market, in which a hexamethylenetetramin-potasium hydrogen phthalate mixing agent is added as a buffer to the resultant work, is dissolved in 40ml water to prepare a reagent. 10ml the reagent is added in sample water, and after 50ml is determined and is left standing for about 1hr, a measurement is made on an absorbance by a wavelength of 620nm to analyze fluorine ion for determination. This eliminates the need for the use of acetone being harmful to a human body, shortens a time required for preparation of a reagent, and also permits the improving of analysis precision.