PURPOSE: To produce an immobilized protease having improved thermal stability and pH stability industrially and at a low cost, by incorporating an alkali protease and/or neutral protease in a specific wt% into fibroin.
CONSTITUTION: An aqueous solution of fibroin is incorporated with an aqueous solution of an alkali protease and/or neutral protease adjusted to pH5W5.5. The fibroin and the protease are salted out and precipitated by the use of an inorganic and/or organic salt, and the resulted precipitate is washed with water and dried to prepare an immobilized protease. The aqueous solution of fibroin is obtaned by dissolving sericin scoured and removed from silk, e.g. raw silk or cocoon, in an aqueous solution of an alkali metal salt, or further dialyzing and desalting the solution, and the concentration of the equeous solution is preferably 2W20wt%.