To obtain a styrene-based polymer where a branching parameter can be controlled in a wide range and the parameter and a molecular, weight are independently controllable by the radical polymerization of a styrene-based monomer in the presence of both a multifunctional vinyl compound and a multifunctional chain transfer agent.

This styrene-based polymer is obtained by polymerizing a styrene-based monomer (e.g. styrene, α-methylstyrene or the like) in the presence of (A) (preferably 1-700 ppm) a multifunctional vinyl compound (e.g. divinylbenzene or the like) and (B) (preferably 10-2,000 ppm) a multifunctional chain transfer agent (e.g. ethylene glycol bisthioglycolate or the like) (respectively based on the weight of a styrene-based monomer) and, if needed, (C) an initiator such as a peroxide (e.g. cyclohexanone peroxide or the like), an azo compound [e.g. 2,2'-azobis(2-methylbutyronitrile) or the like].