Foamable modified polystyrene resin particles (E) comprising a particle of a modified polystyrene resin (C) containing a blowing agent (D), wherein the modified polystyrene resin (C) comprises conjugated diene polymer rubber particles (B) dispersed uniformly throughout a polystyrene resin (A) and when the foamable modified polystyrene resin particle (E) is expanded, there is substantially no deformation of the rubber particles (B) before and after the expansion. A foamed article of a modified polystyrene resin which has a cell membrane of the modified polystyrene resin comprising a polystyrene resin and conjugated diene polymer rubber particles dispersed uniformly throughout the polystyrene resin and has a fusion rate of not less than 50%, wherein the rubber particles maintain substantially spherical form in the cell membrane. According to the present invention, the foamed article of the modified polystyrene resin having break resistance and fusion rate which are equal to those of a high impact polystyrene foamed article can be provided at low cost.