To provide a thermal decomposition apparatus that can inhibit the intensive deposition of carbonized matter caused by the temperature rise on the tubular heat transfer surface of the heat exchanger set in the thermal decomposition tank.

In the thermal decomposition apparatus equipped with a thermal decomposition tank 7 for thermally decomposing plastic solution, the heat exchanging part 10 is set inside the thermal decomposition tank 7 for heating the plastic solution, the heat exchanging part 10 has a tube 35 equipped with fins 35b inside, and the heating medium flows through inside the heat exchanging part 35 to heat the plastic solution. In this case, the pitch of the fin 35b is made coarser at the flowing-in side than at the flowing-out side in the flow of the heating medium.