In a process for producing a conjugated diene copolymer, the improvement in which when copolymerizing a conjugated diene monomer with an ethylenically unsaturated nitrile monomer, at least one compound selected from a bifunctional peroxide represented by formulaR.sub.1 --OO--R.sub.2 --OO--R.sub.3 (I)wherein R.sub.1, R.sub.2 and R.sub.3 each denote a hydrocarbon group, anda bifunctional peroxycarbonate represented by formula ##STR1## wherein R.sub.4, R.sub.5 and R.sub.6 each denote a hydrocarbon group, said bifunctional peroxide and peroxycarbonate being soluble in said monomers, is used as a polymerization initiator.