The gene encoding cellulose-binding domains (CBDs) from Clostridium stercorarium xylanase A was joined to the egIV gene encoding endoglucanase IV (EGIV) from Ruminococcus albus. The hybrid protein (EGIV + CBD) expressed from this fusion gene in Escherichia coli acquired the ability to adsorb onto insoluble celluloses such as ball-milled cellulose (BMC). EGIV + CBD was more active toward BMC at low concentrations than the parental enzyme, EGIV, although there was no difference in the catalytic properties between them toward carboxymethyl cellulose. This result indicates that the addition of the CBDs to EGIV enhances enzyme activity on the insoluble cellulose by concentrating the catalytic domain on the substrate surface.