PURPOSE: To avoid deterioration of the function of a catalyst and at the same time avoid block of the catalyst due to condensation of a polymer and a gas of a component with high boiling point by almost completely eliminating the temperature difference in the inner and outer circumferential parts of a circular contact reaction route and adjusting the introducing amount of a thermally decomposable gas.

CONSTITUTION: A thermally decomposable gas generated in a thermally decomposing tank 1 is introduced into a leading pipe 31 upward from a lower side and cooled by a thermally decomposable gas cooling apparatus 7 installed as to face to an aperture part of an upper end of the leading pipe 31, so that only a gas, which is a component having lower boiling temperature than a set temperature of the thermally decomposable gas cooling apparatus 7, is turned back toward a circular contact reaction route 32 and reacted by being brought into contact with a catalyst with which the contact reaction route 32 is filled. In this case, the catalyst is heated by the thermally decomposable gas rising in the leading pipe 31 and an external heating means 8 installed in the outer circumference of a cylindrical shell 30.