To provide a fiber-treating agent with which cellulosic fibers can efficiently be bio-washed, and to provide a method for treating the fibers.

The fiber-treating agent includes a thermophilic endoglucanase as an active ingredient. The treating method comprises making both a thermophilic endoglucanase and a thermophilic amylase to act on cellulosic fibers to simultaneously subject the cellulosic fibers to both a desizing treatment and a bio-washing treatment.