PURPOSE: The titled composition suitable in the field of light electrical appliance, etc., having improved quality total balance in impact strength, gloss, etc., obtained by blending a rubber modified polystyrene having a specific microstructure with a specific amount of a specified polydimethylsiloxane.

CONSTITUTION: A rubber modified polystyrene in which a rubber-like polymer is dispersed in a particulate state of 0.2W1μ, preferably 0.2W0.7μ average particle diameter and which has a ratio of a gelatinous component and a rubber component of 2W3.3, preferably 2.3W3 is blended with 0.005W0.8wt% polydimethylsiloxane which has a structural unit shown by the formula and 10W10,000 centistoke viscosity.