A polyvinyl chloride resin having an average particle diameter of not less than 150 &mgr;m and porosity at 31-1,011 psi of not less than 0.15 cc/g is suspended in an aqueous medium and chlorinated by blowing gaseous chlorine into the aqueous suspension. In this way, the floating resin formation during chlorination can be prevented and a chlorinated polyvinyl chloride resin excellent in free flow property and showing good resistance to initial discoloration and good thermal stability in heating and molding processes can be produced.