To provide a high efficiency regenerative heat exchanging reactor in which the quantity of heat to be discarded can be reduced.

High temperature fluid to be processed discharged from a reactor 1 is introduced through a changeover valve 6 to a regenerative heat exchanger 4 where the temperature of the fluid is lowered by imparting heat to other fluid. Subsequently, it is separated into a production substance, an intermediate production substance and a nonreacted substance by means of a separator 5. The production substance is taken out to the outside of the system and the intermediate production substance and nonreacted substance are transferred through a pump 2 and a changeover valve 7 to a regenerative heat exchanger 3. In the regenerative heat exchanger 3, the intermediate production substance and nonreacted substance are heated again up to the vicinity of reaction temperature by receiving stored heat and transferred to the reactor 1 where they are used for reaction. When the operation is repeated, nonreacted substance and intermediate production substance in high temperature fluid close to the reaction temperature discharged from the reactor 1 are recirculated to the reactor 1 at a temperature close to the reaction temperature.