PURPOSE: To obtain the title treating agent containing a specific amino-modified silicon as a main ingredient and capable of providing excellent flexibility and stretchability to a fiber product without causing change to yellow or discoloration by treating the fiber product.

CONSTITUTION: A reaction product (a silicone based fiber treating agent) of an organopolysiloxane containing a primary and/or secondary amino group expressed by the formula [R is 1-10C monovalent hydrocarbon; R1 is H or monovalent hydrocarbon; A is R, OR, OH, Q(NHCH2CH2)aNHR' (Q is divalent hydrocarbon); a is 0-4] with a lactone compound (preferred example: lactone having 2-8C alkylene group) reactive with the amino group is emulsified and the resultant emulsion is applied to a natural fiber or synthetic fiber by a dipping method to provide flexibility and stretchability to a fiber product without causing change to yellow or discoloration by heat and passage with time.