A chlorinated vinyl chloride-based resin composition in which from 2 to 9 parts by weight of an MBS resin and from 0.5 to 3 parts by weight of chlorinated polyethylene are blended to 100 parts by weight of a chlorinated vinyl chloride-based resin with a chlorine content of from 62 to 70% by weight obtained by post-chlorinating a vinyl chloride-based resin, which can provide an injection molding product such as a joint of excellent heat resistance, impact strength, moldability and surface property.