To provide an anaerobic flat plate interface bioreactor capable of enabling an interface bioreactor suitable for a microorganism conversion reaction by an aerobic microorganism, as applicable to an anaerobic microorganism.

This bioreactor is characterized by filling a flat plate type interface bioreactor obtained by arranging a hydrophilic immobilized flat plate carrier 5 containing a nutrition source and water in its inside and on the surface of which an anaerobic microorganism membrane exists at the bottom part of a bat 7 in a gas collection bag 2, making the inside of the bag as anaerobic condition by de-aerating and substituting an inert gas and bringing an anaerobic microorganism propagating on the surface of immobilized flat plate carrier in contact with an organic solvent layer 3 containing a hydrophobic substrate.