To obtain ethylenediamine zinc phosphate which is a material having excellent flame retardancy, and is useful as a flame retardant for flame retardant resin compositions such as various electric parts, parts for cars, construction materials, and electric wire/cables.

This compound, which is shown by the formula Zn2P2O8C2N2H10, has at least spacings shown in the table in X-ray diffraction pattern, and has a particle size of 4 μm or smaller, is obtained by mixing an aqueous solution of zinc phosphate having a (phosphoric acid/zinc) molar ratio of 1.0-2.0 and an aqueous solution of ethylenediamine at an (ethylenediamine/zinc) molar ratio of 0.5-1.0, adjusting the pH of the resultant mixture to 3-11 at 40°C or lower, aging at 40°C or lower, preferably, for 2-24 h, crystallizing, collecting crystals, washing the obtained crystals, followed by drying at 60-250°C for several to several tens hours. Zinc sulfate, zinc nitrate, zinc chloride, and the like are cited as zinc compounds to be used. An aqueous solution of zinc phosphate at several tens to several thousands mmol/l is enough.