The present invention can reduce the amount of carbon dioxide exhausted during the production of an organic substance from waste. The apparatus 1 for producing an organic substance, comprises a synthesis gas generation furnace 11 and a fermenter 13. The synthesis gas generation furnace 11 generates a synthesis gas by partial oxidation of waste. The fermenter 13 contains microorganisms which produce an organic substance from the synthesis gas. The fermenter 13 comprises a first fermenter unit 13a and a second fermenter unit 13b.