A closed heat-decomposing appliance being an appliance for horizontally or slantly inserting heating section into a heating device to heat, decomposing the inner organics in the presence of oxygen gas, taking out from the heating device and cooling, then introducing the absorbing liquid to absorb the testing components, comprising a heating section with one side closed and other side having common ground portion, screw portion or O-ring-mounted portion and a closed introducing section that allows to connect to this heating section by common ground portion, screw portion or O-ring via O-ring-mounted portion and has cock or valve as a mechanism for closing and introducing the absorbing liquid to absorb the testing components from outside after heat-decomposition, or has packing or septum to introduce the absorbing liquid with needle pipe as well.