Disclosed is a fiber-treating agent composition comprising an oil-in-water emulsion wherein composition (A) comprising the following component (b) is emulsified with the following component (a): a polymer compound of a polysaccharide derivative, and component (b): a polyether-modified silicone having an HLB value of larger than zero to 7 or smaller; wherein the mass ratio of the component (a) to the component (b) in the composition, that is, the component (a)/component (b), is 9/100 to 30/100.