A method for suppressing a reduction in an endoglucanase activity in the presence of a surfactant, characterized by modifying a protein having the endoglucanase activity in which the N-terminus is an amino acid other than pyroglutamic acid, to a protein having the N-terminus of pyroglutamic acid, is disclosed. Further, a modified protein having an endoglucanase activity wherein the N-terminal amino acid is converted into pyroglutamic acid by an amino acid modification, a polynucleotide encoding the protein, an expression vector comprising the polynucleotide, a host cell transformed with the expression vector, and a process for producing the protein by cultivating the host cell, are disclosed.