Disclosed is a positively charged antimony pentoxide sol, comprising colloidal particles of antimony pentoxide sol coated on their surfaces with at least one of trivalent and tetravalent metals, the content of the metals being 1 to 50% by weight in terms of the metal oxide based on antimony pentoxide in the antimony pentoxide sol.When the positively charged antimony pentoxide sol of this invention is mixed with a cationic resin emulsion or an aqueous solution of a cationic water-soluble polymer, no agglomeration occurs and also, when it is added into an aqueous solution of an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid, etc., or an organic acid such as acetic acid, propionic acid, oxalic acid, maleic acid, etc., an aqueous solution of a metal salt or amine salts of these acids or an organic solvent such as an alcohol, no agglomeration occurs but it is well dispersed therein, and therefore it can be applicable for fields in which the antimony pentoxide sol of the prior art cannot satisfactorily be used.