To provide a method for producing lanthanum hexaboride-containing composite particles which are capable of forming a formed product having sufficiently high transparency and which are excellent in weather resistance, by a simple operation without calcination treatment at high temperature, and a method for producing a formed product using it. Also provided is a method for producing composite particles, which involves: reacting at least one silica precursor selected from a tetraalkoxysilane, its hydrolysate and its condensate, in the presence of lanthanum hexaboride particles, a volatile base, water and an organic solvent to obtain a first reaction mixture, and reacting the first reaction mixture with at least one silicon compound selected from an amino-modified silicone, an alkylsilane and an aminosilane, or the silicon compound and the silica precursor added, to obtain a second reaction mixture containing lanthanum hexaboride-containing composite particles.