To obtain a multiphase copolymer-containing resin composition having excellent heat resistance, shock resistance and moldability.

This composition comprises (I) a graft copolymer mixture containing a multiphase copolymer which contains at least three phases of an inner phase (a) consisting of diene rubber polymer particles having an average particle diameter of 0.1-10 μm, an outer phase (b) consisting of a copolymer composed mainly of an aromatic vinyl monomer (i) and a cyanide vinyl monomer (ii), and further dispersed particles (c) mainly composed of (i) and (ii) which are dispersed in the inner phase (a) as constituting elements, (II) a hard polymer phase composed mainly of (i) and (ii), and (III) a maleimide-based copolymer.