PURPOSE: To prepare the titled compound by a peroxobromination method from the corresponding alkylidenediphenol in high quality with high yield, even when the mother liquior is recycled, (by washing the organic phase with an aqueous alkali metal sulfate solution and the water before the crystallization, and separating the water every washing.

CONSTITUTION: The corresponding 4,4'-alkylidenediphenol is brominated by using Br2, a HBr3 aqueous solution or HBr aqueous solution, and HBr aqueous solution used or generated in this reaction, is oxidized by H2O2 in the presence of water-insoluble organic solvent. After the reaction, a temperature of the reacted mixture is elevated to dissolve the titled compound, the water phase and the organic phase are separated, then the organic phase is washed with the aqueous alkali metal sulfite solution and the water at least each one time, and the washed water phase is separated every washing. Further the organic phase is cooled to crystallize the titled compound, and the crystalline and the mother liquor are separated, to effectively obtain the titled compound usuful as a flameproof agent for plastic, in high purity without reducing the yield even when the motor liquor is recycled several times, without increasing the cost so much.