Disclosed is a method for producing a rubber-modified styrenic resin. The method includes the steps of (a) producing a rubber solution by dissolving a rubber polymer in a vinyl monomer; (b) producing a rubber composition by continuously supplying the above rubber solution in a first reactor to polymerize the rubber polymer with the vinyl monomer at a reaction temperature of 120°-145° C. to perform phase inversion of the rubber polymer, thereby producing the polymer of conversion ratio of 10-40 wt %, and realized through 50-99% of rubber particles having an average particle diameter of 0.1-0.5 &mgr;m, and 1-50% of rubber particles having an average particle diameter of 1-5 &mgr;m; (c) performing polymerization until conversion ratio reaches 70-90% by continuously supplying a reactant solution produced in the first reactor in a second, third and fourth reactor; and (d) separating the polymer containing a rubber component from unreacted monomer by continuously supplying a reaction solution resulting from the fourth reactor to a devolatilizer.