To obtain an interfacial bioreactor having good efficiency, in which defects which conventional various interfacial bioreactors have are dissolved.

This bioreactor is obtained by superposing plural flat plate type interfacial bioreactors in high layers, connecting organic solvent layers of these interfacial bioreactors to each other and circulating the solvent. The flat plate type interfacial bioreactor is obtained by locating a hydrophilic immobilization plate-like carrier in which nutrient and water are contained and a proliferated microorganism film is attached and fixed on the surface on the bottom of the bioreactor and locating the proliferated microorganism film thereon, providing an organic solvent layer containing a hydrophobic substrate thereon, bringing a proliferated microorganism film into contact with the organic solvent layer to convert the hydrophobic substrate by a microorganism.