Alkali metal salts or/and alkaline earth metal salts of cyclic phosphorus compounds expressed by the general formula ##STR1## (wherein X.sub.1 to X.sub.8 each represent hydrogen, halogen, cyano group, acyl group, alkyl group, halogen-substituted alkyl group, aryl group, halogen-substituted aryl group or aralkyl group) are provided. These metal salts impart a specific flame retardancy to organic high molecular weight compounds prepared from alkylidenebisphenols as a raw material, without lowering the characteristic properties of substrates.