PURPOSE: To obtain the subject composition, containing phenylglyoxal and capable of imparting excellent elasticity such as tenseness and stiffness to hair and sustaining the effects for a long period by treatment one time even by repeating washing of hair, etc.

CONSTITUTION: A keratinous fiber treating composition containing 0.01-10wt.%, especially 0.1-5wt.% phenylglyoxal. A method for, e.g. applying the composition to hair, then allowing the hair to stand for a prescribed time and washing off the composition, etc., is cited as a method for using the above-mentioned composition. The hair may be heated to a prescribed temperature during the period of allowing the hair to stand. If the hair is treated after pretreating the hair with a reducing agent, especially a pretreating agent containing thioglycolic acid or its salt, bonding of the phenylglyoxal to the keratinous fiber is promoted to further improve the elasticity imparting effects.