A glass fiber-reinforced synthetic resin composition is proposed which is particularly suitable as a molding material of chassis of electric and electronic instruments to satisfy the various requirements in this particular application. The resin composition comprises: (A) 100 parts by weight of a copolymeric resin of styrene and maleic anhydride or a derivative thereof, of which the molar fraction of the moiety of maleic anhydride or a derivative thereof is in the range from 3 to 20%; (B) from 1 to 30 parts by weight of a styrene-based thermoplastic elastomer, of which the weight fraction of the moiety of styrene is in the range from 5 to 50%; (C) from 5 to 80 parts by weight of glass fibers, (D) from 5 to 30 parts by weight of a bromine-containing organic compound as a flame retardant; and (E) from 2 to 15 parts by weight of antimony trioxide.