NOVELTY - An organic substance is oxidized using oxygen discharged from metal oxide catalyst having oxidized metallic valence to form a metal oxide catalyst having reduced metal valence, and oxidized organic material, and simultaneously, oxygen released from water oxidizes the metal oxide catalyst having reduced metal valence to regenerate metal oxide catalyst having oxidized metal valence. The organic substance is processed in hydrothermal conditions using redox cycle of solid electrolyte metal oxide catalyst.    USE - Processing of organic substance e.g. aldehyde, sludge, lignin, plastics waste material and biomass waste (all claimed). Uses include but are not limited to paper, livestock excrement, foodstuff waste material, construction waste material, black liquor, sewage sludge, organic refuse, rice straw, woodland surplus material, starch-type crops, etc.    ADVANTAGE - The method enables processing of organic substance, efficiently obtains lightened product, and suppresses heavy-izing and coke formation by thermal absorption oxidation reaction of organic substance. The waste heat from the low-temperature source is efficiently recovered as bond energy of the oxidized product. The system used for processing organic substance has improved efficiency.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) contact reaction apparatus (1), which has discharge portion (6), and contact reaction device (2) having reaction catalyst layer including inlet (4) for organic substance, inlet (5) for water and metal oxide catalyst, and oxidized organic substance reaction products;    (2) system, which has contact reaction apparatus, separation tank for separating gaseous product and product mixed solution containing hydrogen gas as main component, collection tanks for gaseous product and product mixed solution, and a system which performs pressure control for taking out product to collection tank from the separation tank based on the production flowing amount of gaseous product and product mixed solution; and    (3) exhaust-heat recovery method from low-temperature source, which involves processing organic substance, and collecting waste heat from redox cycle of metal oxide catalyst as bond energy of the product.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the contact reaction apparatus.    Contact reaction apparatus (1)    Contact reaction device (2)    Zone containing reaction catalyst layer including metal oxide catalyst (3)    Inlet of organic substance (4)    Inlet of water (5)    Discharge portion of oxidized organic substance (6)    Discharge portion of hydrogen gas (7)