To provide a liquid additive adding method for obtaining a styrene resin composition excellent in extrusion stability and enhanced in impact strength, heat resistance, molding processing flowability and appearance with high productivity.

In a method for adding a liquid additive (c) to a styrenic resin (a)/polyphenylene ether resin (b) compsn., a twinscrew extruder having a main feed opening part at one place and side feed possible supply opening parts at a plurality of places so as to differentiate the distance from a leading end part is used and the liquid additive (c) is divided so that the viscosity ratio ηN+1/ηN of the shearing viscosity ηN (poise) of a resin before the addition of (c) at the N-th supply opening part successively counted from the supply opening part near to the main feed opening part and the shearing viscosity ηN+1, of the resin before the addition of (c) at the next near the (N+1)-th supply opening part is 0.01-1.0.