A novel diester compound of the formula (I): ##STR1## wherein each of R.sub.1 and R.sub.2 represents a group of the formula (II): ##STR2## wherein X is bromine atom; Y at least one group selected from the group consisting of alkyl groups having 1 to 4 carbon atoms, alkoxy groups having 1 to 4 carbon atoms, hydroxyl group and primary, secondary or tertiary amino groups; p an integer of 0 to 4; q an integer of 1 to 4; r an integer 1 to 4 and (q+r) is equal to an integer of not more than 5, is found to be capable of imparting excellent flame-retardancy to various inflammable polymeric materials such as polystyrene, polyester, polyamide, polyphenylene oxide, etc. without deleterious effects on other physical properties of such polymeric materials.