in a process for producing a high impact styrene resin by continuously polymerizing a styrene monomer in bulk in the presence of a rubbery polymer , the improvement wherein a polymerization line is used which is comprised of an initial - stage polymerization line following a material feed line and consisting of one or more tubular reactors having fixedly set therein a plurality of mixing elements having no moving parts , a main polymerization line following the initial - stage polymerization line and consisting of one or more tubular reactors having fixedly set therein a plurality of mixing elements having no moving parts , and a recycle line which branches between the initial - stage polymerization line and the main polymerization line and returns to the initial - stage polymerization line , and a minor part or a major part of a flow of the initial - stage polymer solution leaving the initial - stage polymerization line is recycled through the recycle line while the remaining non - recycled initial - stage polymer flow is polymerized in the main polymerization line .