At least an equivalent amount of concentrated hydrochloric acid is added to antimony trioxide containing a substance emitting alpha rays, and they are stirred. Any undissolved residue is removed to prepare an aqueous solution of antimony chloride. Water is added to the solution, or a distillate obtained by distilling it at a temperature of at least 135.degree. C. The quantity of the water is at least 10 times by volume as much as the solution, or at least 20 times by volume as much as the distillate. The solution or distillate and the water are stirred at a temperature of at least 60.degree. C. to cause precipitation of antimony trioxide, and the precipitate is separated by filtration. The precipitate is washed with at least 10 times by weight as much warm water having a temperature of 60.degree. C., and dried. This antimony trioxide has an alpha-ray strength not exceeding 0.01 C/cm.sup.2 per hour, does not contain more than 1 ppm of any impurities such as arsenic, lead or soluble chlorine, and has a large average particle size.