Lipase derived from Rhizopus delemar was entrapped in the silk fibroin membrane with the methanol-immersion treatment for 10 min. No leakage of the immobilized enzyme was detected by the method using the enzymatic assay. Thermal effect on the activity of immobilized lipase was studied in detail. The optimum temperature for immobilized enzyme (50°C) was higher than that for the free one (20°C). The lipase was much stabilized by the immobilization with silk fibroin since the inactivation constant, which was determined from the measurement of inactivation process at 50°C, was less than 1/40 of that of the free enzyme. The optimum pH shifted to alkaline side as compared to that of the free one. On the basis of several properties concerning the immobilized lipase and dissolubilization state of the substrate, tributyrin, observed by 13C NMR measurement, it was concluded that origin of the stabilization of lipase with silk fibroin was mainly the formation of the unique structure such as Silk II type crystalline in the silk membrane. The lipase-immobilized silk fibroin membrane generated membrane potential with the immobilized-enzyme reaction in the membrane.