PURPOSE: To continuously obtain high-purity styrene monomer in high yield at a low cost by adding a polymerization inhibitor to a styrene resin such as waste styrene foam, depolymerizing the polymer and passing the obtained crude styrene monomer through a partial condenser.

CONSTITUTION: 100 Pts.wt. of a styrene resin (preferably homopolymer) such as waste styrene foam is charged together with 0.1-0.5 pts.wt. of a polymerization inhibitor (e.g. p-benzoquinone) into a melter 10 equipped with a stirrer, stirred, transferred to a reactor 20 and depolymerized at about 330-340°C to obtain crude styrene monomer. The monomer is transferred to a partial condenser 30, blown into the vessel through an inlet port 33 and cooled to about 300°C in the course of ascending in the column. The obtained high-purity styrene monomer is extracted through an outlet 32 and passed through a condenser 35 and a total condenser 40. It is especially preferable to continuously produce styrene monomer from the styrene resin by extracting a prescribed amount of the raw material from the depolymerization reactor 20 at regular intervals and replenishing the reactor with new raw material.