The fiber treatment agent of the present invention includes a volatile solvent and one polymer or a mixture of two or more polymers soluble in the volatile solvent. The solubility of one or more of the above polymers in water at 25°C is less than 50 mg/100 g. The content of one type of the above-mentioned polymer or a mixture of two or more types of the above-mentioned polymer is 3% by mass to 15% by mass. The yield stress of one type of the above polymer or a mixture of two or more of the above polymers at 25°C is 0.6 MPa or more. In addition, the contact angle between one type of the above-mentioned polymer or a mixture of two or more types of the above-mentioned polymer and water is 90 degrees or more.本發明之纖維處理劑包含揮發性溶劑、及可溶解於該揮發性溶劑之1種聚合物或2種以上之聚合物之混合物。1種上述聚合物或2種以上之上述聚合物於25℃下之對水之溶解度未達50 mg/100 g。1種上述聚合物或2種以上之上述聚合物之混合物之含有比率為3質量%以上15質量%以下。1種上述聚合物或2種以上之上述聚合物之混合物於25℃下之降伏應力為0.6 MPa以上。並且，1種上述聚合物或2種以上之上述聚合物之混合物與水之接觸角為90度以上。
