To provide an interface bioreactor suitable for a microorganism conversion reaction by an aerobic microorganism, as applicable to an anaerobic microorganism.

This interface bioreactor is constituted by arranging a hydrophilic immobilized plate like carrier containing a nutrition source and water in its inside and on the surface of which an anaerobic microorganism membrane exists in a vertical direction, and bringing an anaerobic microorganism propagating on the surface of immobilized plate like carrier in contact with an organic solvent layer containing a hydrophobic substrate to perform a microorganism conversion reaction of the hydrophobic substrate. In this case, it is characterized by removing each of oxygen dissolved in the aqueous phase of the immobilized plate like carrier, oxygen dissolved in the organic solvent layer contained in the hydrophobic substrate and oxygen existing in the dead space at the inside of the interface bioreactor, and substituting an inert gas to make it a completely anaerobic condition.