PURPOSE: To satisfactorily trap reaction products, especially thermal decomposition products by forming the apparatus of a vertical decomposition tube externally heating decomposition products of a solid sample and arranging a means for trapping by a trapping device consisting of two vertical concentric tubes.

CONSTITUTION: A decomposition tube 1 is equipped vertically in a furnace 16 for heating the sample 5 and a region 2 of decreased diameter is formed at the down end. By the way, each of a first gas introducing hole 3 at the top end of the decomposition tube and a second gas introducing hole 4 for supplying an inert shield gas at the bottom of the region 2 of decreased diameter is positioned. And a sample port 6 for collecting the sample 5 at the top of the decomposition tube 1 is attached at the bottom end of a rod 7. Further the trapping device consists of two concentric tubes 11, 12 which are in inner side or outer side, a introducing hole 14 is positioned at the bottom end of the device. And a outlet end 15 of the inner tube 12 is extended into a resorvoir. On the other hand, a vacuum means is connected to the resorvoir in order to collect the first gas and shield gas.