Provided is a technology of removing 90% or more chlorine by treating an oil fraction having a high Cl content at a high temperature using a solid acid catalyst. The dechlorinated oil fraction may be introduced to a refinery process and converted into a fuel or a chemical product. The solid acid catalyst and the oil fraction having a high Cl content are mixed and then chlorine is removed by a heat treatment at a high temperature. Main impurities such as S, N, and O and Na, Ca, Fe, and the like which may act as a catalyst poison in the catalyst reactions of a refinery process are removed simultaneously with the process of removing Cl.