supplying a pyrolysis fuel oil (PFO) stream containing PFO and a pyrolysis gas oil (PGO) stream containing PGO to a distillation tower as a feed stream (S10), the PFO stream and the PGO stream being discharged from a naphtha cracking center (NCC) process; and

supplying a lower discharge stream from the distillation tower to a combustion chamber for a gasification process to obtain synthesis gas and supplying an upper discharge stream from the distillation tower to a preparation process (SM/BTX preparation process) including a preparation process of styrene (SM preparation process) and a preparation process of one or more of benzene, toluene, and xylene (BTX preparation process) to obtain aromatic hydrocarbons (S20),

wherein a ratio of a flow rate of the upper discharge stream from the distillation tower to a flow rate of the feed stream to be supplied to the distillation tower is 0.01 to 0.2, and

wherein a kinematic viscosity of the lower discharge stream from the distillation tower at the time of supply to the combustion chamber is controlled to have 300 cSt or less.