PURPOSE: To provide a process for producing a styrene polymer having high molecular weight and capable of essentially dispensing with a monomer removing process because very small amount of the monomer remains after polymerization and molecular weight distribution is narrow even without carrying out pre-polymerization.

CONSTITUTION: A styrene monomer is compounded with a thermal polymerization initiator and a photopolymerization initiator to obtain a composition for polymerization. The composition is supplied into an extruder and heated up to a temperature higher than the decomposition temperature of the thermal polymerization initiator to carry out thermal polymerization. When the polymerization conversion ratio of the styrene monomer reaches 40-90%, the obtained thermal polymerization product is subjected to ultraviolet irradiation to carry out photopolymerization before or after extrusion from an extruder.