and may at times detract therefrom the ranges of the calcium ion and hydroxyl ion concentrations mentioned above include the respective saturation concentrations this invention further requires the cellulose fibers which have undergone the foregoing treatment to be immersed in an aqueous solution containing calcium ions and phosphoric acid ions in this immersion the concentration of calcium ions is in the range of 0 002 mol liter to 0 08 mol liter and that of phosphoric acid ions in the range of 0 001 mol liter to 0 04 mol liter the temperature of retention in the range of 20 u00b0 to 50 u00b0 c and the period of retention in the range of one to 14 days if the temperature or the period of the retention of the cellulose fibers in the aqueous solution falls short of the lower limit of the range concerned or the concentration of the calcium ions or the phosphoric acid ions in the aqueous solution is unduly low the desired coating layer of calcium phosphate compound is not formed conversely increasing the temperature or the period of the retention beyond the upper limit of its range serves no useful purpose because it does not proportionately add to the effect of the immersion and may at times detract therefrom when the concentration of the calcium ions or the phosphoric acid ions is unduly high it is difficult to form a uniform coat of calcium phosphate compound on the cellulose fibers the term 34 calcium phosphate compound.