Abstract: A fiber treatment agent composition comprising the following components (A) and (B), wherein a content of the component (A) is 0.05% by mass or more and 5.0% by mass or less, the content of the component (B) is 0.05% by mass or more and 10.0% by mass or less, and a mass ratio of the component (B) to the component (A), (B)/(A), is 150 or less: (A) a self-crosslinking compound; and (B) one or more color deepening agents selected from the group consisting of a nonvolatile silicone, a polyhydric alcohol polymer, a fatty acid ester, and an acrylic resin (excluding agents corresponding to the component (A)).