NOVELTY - Molecular weight reduction of organic substance involves generating mixed gas (g) which contains hydrogen and carbon dioxide by shift reaction, adding excess water vapor to waste gas (g0) containing carbon monoxide, performing shift reaction, contacting mixed gas (g) with organic substance, reforming and rendering low-molecular weight organic substance, inducing flow medium (m) and char as by-produced to rotary kiln combustion furnace, heating flow medium (m) and char by air combustion, and circulating heated flow medium (m) in fluidized-bed molecular weight reduction furnace.    USE - Molecular weight reduction of organic substance e.g. plastics, oil-impregnated sludge, waste oil and biomass (all claimed) used for converting gaseous fuel.    ADVANTAGE - The method enables efficient and economical molecular weight reduction of organic substance with high freedom degree of combustion and without melting organic substance.    DETAILED DESCRIPTION - Molecular weight reduction of organic substance involves generating mixed gas (g) which contains hydrogen and carbon dioxide by shift reaction, adding excess water vapor which is not consumed by shift reaction to waste gas (g0) containing carbon monoxide generated in the metallurgical furnace, performing shift reaction, contacting mixed gas (g) with organic substance, reforming and rendering low-molecular weight organic substance having true density of 4-8 g/cm3, inducing flow medium (m) and char as by-produced generated during molecular weight reduction reaction to rotary kiln combustion furnace, heating flow medium (m) and char by air combustion in rotary kiln combustion furnace, and circulating heated flow medium (m) in fluidized-bed molecular weight reduction furnace (2). At least one portion of flow medium (m) contains granular material (f) containing chromium, iron and/or nickel. An INDEPENDENT CLAIM is included for molecular weight reduction system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the molecular weight reduction system of the organic substance. (Drawing includes non-English language text)    Reaction apparatus (1)    Fluidized-bed molecular weight reduction furnace (2)    Extraction unit (3)    Rotary kiln combustion furnace (4)    Supply unit (5)