Disclosed is a flame retarded styrene-based resin composition containing a brominated flame retardant and an additive capable of enhancing the flame retardancy of the shaped article of the composition even in a very small amount of addition. The composition comprises (A) a styrene-based resin, (B) 0.1 to 30 parts by weight per 100 parts by weight of the resin (A) of a brominated flame retardant having a bromine content greater than 50 wt %, and (C) 0.01 to 0.2 parts by weight per 100 parts by weight of the resin (A) of a metal phthalocyanine selected from iron, manganese, cobalt, nickel, platinum or paradium phthalocyanine.