The present invention provides an apparatus and a method which can be used for producing an organic substance effectively from a synthesis gas obtained from a waste incinerator. The apparatus 1 for producing an organic substance from waste, comprises a synthesis gas generation furnace 11, a fermenter 13, and a nutrient feeder. The synthesis gas generation furnace 11 generates a synthesis gas by partial oxidation of waste. The fermenter 13 contains a microorganism which produces an organic substance from the synthesis gas. The nutrient feeder 12 feeds a solid or liquid nutrient to the fermenter 13 when an amount of the synthesis gas supplied to the fermenter 13 is insufficient.