A composite metal hydroxide is produced by reacting magnesium-containing aqueous solution (X) including water soluble zinc compound, in which magnesium ion concentration is 0.01 mol/liter, with alkaline material (Y) at a specific reaction equivalent ratio. Further, a composite metal hydroxide represented by the following general formula (1) is produced by hydrothermally treating thus obtained composite metal hydroxide within a temperature range of 100 to 200 DEG C for aging in chlorine-containing aqueous solution having a specific chlorine ion concentration. Mg(1-x)Znx(OH)2 (1) in which x indicates a positive number within a range of 0.003 ≤ X ≤ 0.1. A uniform metallic solid solution can be obtained as a composite metal hydroxide by the above series of processes. Further, its crystal shape can be controlled thereby so as to restrain occurrence of secondary aggregation. A flame retardant high-molecular composition, obtained by including thus obtained composite metal hydroxide into a high-molecular composition, exerts high flame retardancy and shows superiority in mechanical strength.