A nanofiltration membrane, UTC-20, was found to have a high rejection ratio to dissociable coenzymes such as NAD, NADP, and ATP irrespective to the ionic strength of the solution, By using UTC-20, a nanofiltration membrane bioreactor (NFMBR) was constructed for the continuous stereospecific reduction of 2-ketoglutarate to L-glutamate with NADH regeneration, In die NFMBR a conjugated system of glutamate dehydrogenase and glucose dehydrogenese were coimmobilized and the substrate solution containing a low concentration of NAD was supplied continuonsly, Because of rejection by the membrane, NAD was partially immobilized and concentrated in the reactor so that a high regeneration number of NAD (similar to 10000) was obtained with a good conversion (similar to 0.9) and productivity (similar to 120 g/l/d). NFMBR is expected to be applicable to a bioreactor involving dissociable coenzymes in general.