PURPOSE: To obtain flame retardant for PS not causing the deterioration of the properties of PS and having no fear of injuring equipment such as molds or molding machines, by mixing tris(2,3-dibromopropylene) isocyanurate with Sb2O3.

CONSTITUTION: 50W100pts.wt. Sb2O3 and, optionally, a colorant and a binder are added to 100pts.wt. tris(2,3-dibromopropylene) isocyanurate, and the mixture is pelletized to obtain a flame retardant for PS. Said flame retardant is added to PS so that the amount of the tris(2,3-dibromopropylene) isocyanurate may be 1% or above.