To provide a measurement system that has no problems of wear, rupture, and the like by the insertion of a protection tube, and hardly lose measurement precision instead of a system for inserting the protection tube that is the conventional direct measurement for the measurement of temperature at the exit of a decomposition furnace for an ethylene manufacturing apparatus.

In the temperature measurement system at the exit of the decomposition furnace for the ethylene manufacturing apparatus, the protection tube incorporating a temperature sensor inside is inserted into a seat section for mounting the temperature sensor protection tube at the bending section of the exit piping in the decomposition furnace. The tip of the protection tube machined to a shape for forming nearly the same surface as the inner wall of exit piping is located at a position for forming nearly the same surface as the inner wall of the exit piping.