An aqueous alkaline solution containing a tin salt dissolved therein is mixed with a zinc compound, and an aqueous solution of an indium salt is added to the mixture. The resultant hydroxide or hydrate containing tin, indium and zinc is treated by heating at a temperature of 110 to 300.degree. C. in the present of water. Then, the resultant product is filtered, dried and treated by heating at a temperature of 300 to 1,000.degree. C. in an air and further reduced at a temperature of 150 to 400.degree. C. under a reducing atmosphere to obtain composite indium oxide particles of zinc oxide and tin-containing indium oxide, which have an average particle size of 5 to 100 nm. The resultant composite particles of zinc oxide and tin-containing indium oxide are suitably used to form a transparent conductive coating film having a UV-shielding effect.