NOVELTY - Thermal decomposition is carried out using dechlorination, thermal decomposition, and combustion fluid beds which are deployed for two circulations. One is a circulation to which heat carrier returns from dechlorination fluid bed (1) to bed (1), passing through thermal decomposition and combustion fluid beds (2,3). The other is a circulation which returns from bed (2) to bed (2), passing through bed (3).    USE - For thermally decomposing mixed waste plastic containing chlorinated plastics, such as PVC, in oil formation.    ADVANTAGE - The thermal decomposition method facilitates process-management and apparatus-maintenance. Since a circulation-type fluid bed which can supplement a dechlorination and thermal decomposition with the heat of combustion of char is built, the process and apparatus are simplified. Autolysis of char is carried out, and smooth running is enabled. A thermal decomposition gas of high heating-value is obtained, which is advantageous for subsequent gas-purification and oil-formation processing.    DETAILED DESCRIPTION - A thermal decomposition method uses three fluid beds, with a common heat carrier, which carry out dechlorination, thermal decomposition, and combustion. The fluid beds are deployed so that two circulations are carried out. One is a circulation to which the heat carrier returns from the dechlorination fluid bed (1) to the dechlorination fluid bed, passing through the thermal decomposition fluid bed (2) and the combustion fluid bed (3). The other is a circulation which returns from the thermal decomposition fluid bed to the thermal decomposition fluid bed, passing through the combustion fluid bed. The circulations distribute a suitable quantity of the heat carrier heated with the combustion fluid bed, to the dechlorination fluid bed and the thermal decomposition fluid bed. The fluid beds are heated to a required temperature, and thermal decomposition is continuously carried out with dechlorination of raw material.    An INDEPENDENT CLAIM is included for thermal decomposition apparatus equipped with dechlorination fluid bed, thermal decomposition fluid bed, and combustion fluid bed. Each fluid bed is connected to the moving bed path (4-7) of sand, which is the automatically-circulated heat carrier.    DESCRIPTION OF DRAWING(S) - The figure shows the explanatory drawing of the thermal decomposition method. (Drawing includes non-English language text).    dechlorination fluid bed (1)    thermal decomposition fluid bed (2)    combustion fluid bed (3)    moving bed path (4-7)