To obtain a styrenic copolymer having narrow molecular weight distribution and thereby containing no ultrahigh molecular weight component and no low molecular weight component by specifying a concentrations of a compound having a vinyl group and a radical scavenger.

This styrenic copolymer has 100,000-500,000 weight average molecular weight(Mw), 50,000-400,000 number average molecular weight (Mn), 1.5-2.5 molecular weight distribution (Mw/Mn) and 0.1-10 branched degree (λ), being a number of branchings per 1,000 units styrene monomer, and about same values thereof are shown in number average molecular weight (Mn), weight average molecular weight(Mw) and Z-averaged molecular weight (Mz). The objective copolymer is preferably obtained by copolymerizing (B) a styrenic compound with (C) a compound having plural vinyl groups in an amount within the range of the formulas 3.0×10-3≤C≤1.0×10-1 and 0.2≤C/D≤4.0 [D is a concentration (mol%) of component C based on the amount of the component B; C is the concentration (mol%) of component A] at 100-160°C polymerization temperature so that the polymerization conversion may be 40-80%.