PURPOSE: To readily impart unique hand and softening effects unobtainable by treatment with each of a single alkali and enzyme, suppress deterioration in tenacity and modify a cellulosic fiber structure by subjecting the cellulosic fiber structure to treatment with the alkali and enzyme in combination at a high temperature under a high pressure.

CONSTITUTION: An aqueous solution of caustic soda in preferably 150-300g/l concentration is applied to a fiber structure (e.g. cotton) containing cellulosic fiber and the resultant fiber structure is directly brought into contact with high-temperature steam at 102-160°C, then subjected to treatment for removing the alkali, further treated with a solution prepared by adding acetic acid to an aqueous solution of cellulase in 0.1-5.0% concentration and regulating the pH to 3-6 at 20-60°C for 0.5-48hr, finally subjected to dipping treatment in an aqueous solution containing 0.1-0.3% caustic soda and 0.3-0.7% sodium thiosulfate and washed with water to carry out weight reduction processing of the cellulosic fiber structure.