To provide a carbonization product while preventing a chlorinated plastic from adhering to each other and from adhering to the inner wall of a decomposition apparatus and while improving the decomposition ratio by mixing a chlorinated plastic with an oil coke powder and decomposing the mixture.

100 pts.wt. chlorinated plastic in irregular, spherical, circular cylindrical, prismatic, or cubic shape having a major axis of 1-30 mm, pref. 2-5 mm, and a thickness of 30 mm or lower is mixed with 3-500 pts.wt., pref. 5-300 pts.wt., still pref. 5-50 pts.wt., oil coke powder having an average particle size of 0.01-2 mm, pref. 0.02-1 mm, under such conditions that the surface of the plastic slightly melts. The resulting mixture is fed to a thermal decomposition oven such as a rotary kiln, a fluidized bed oven, or a vertical oven and is decomposed in an inert gas atmosphere at 250-350°C for 5 min to 5 hr, pref. 10-60 min. The resulting hydrogen chloride gas is absorbed by an aq. alkali soln.