The present invention provides a method for manufacturing a styrene-based polymer having the steps of continuously feeding raw materials into one end of two or more polymerization vessels connected in series; polymerizing a styrenic monomer in the coexistence of a (meth)acrylate ester monomer by using an organic radical generating agent as an initiator by a mass or solution polymerization method of continuous process; and continuously taking out a product from another end of the polymerization vessels, wherein polymerization conditions are controlled in ranges specified by the following items (a) and (b): (a) a composition of monomers to be fed is 60 to 97% by weight of a styrenic monomer(s) and 3 to 40% by weight of a (meth)acrylate ester monomer(s); and (b) a concentration of the (meth)acrylate ester monomer(s) at any part of the polymerization vessels exceeds 1% by weight based on total monomers at any time during the polymerization.