According to the present invention, during or after the process of separating an aqueous solution of hydrochloric acid by-produced from a chlorinated vinyl chloride resin slurry obtained by chlorinating vinyl chloride resin or a copolymer of vinyl chloride and another copolymerizable monomer, hypochlorite is added to the chlorinated vinyl chloride slurry or water-containing resin. Due to this, the concentration of hydrochloric acid in the chlorinated vinyl chloride resin or chlorinated vinyl chloride copolymer resin before drying is reduced, and the problem of removing hydrochloric acid in the subsequent drying treatment is reduced, whereby the problem of corrosion of a drier can be reduced.