A mutated alkaline cellulase derived from a cellulase having the amino acid sequence represented by SEQ ID NO:1 or one having a homology of at least 90% therewith by deleting one or more amino acid residues from the 343- to 377-positions in SEQ ID NO:1 or a region corresponding thereto and then inserting a peptide having from 2 to 15 amino acid residues into the deletion site; and a gene encoding the same. The above alkaline cellulase has an optimum pH value close to the pH value of laundry water (around pH 10.5) and, therefore, is useful as an enzyme for detergents.