In producing chlorinated polyvinyl chloride resin having chlorination degree of 60-73% by weight, wherein polyvinyl chloride resin is suspended in an aqueous medium and chlorine gas is blown into said suspension under beam of a mercury lamp in the temperature range of 40-90° C., an organic peroxide compound having a 10 hour half-life in the range of 40-90° C. is added into the reaction vessel in the ratio of 0.01-1 parts to 100 parts of the polyvinyl chloride resin by weight before the chlorination reaction is started. By this method, the time for chlorination reaction can be shortened without sacrificing the initial coloring and the heat stability, whereby chlorinated polyvinyl chloride can be produced with significantly improved productivity.