Abstract: In one or more embodiments, the present invention relates to a resin composition containing at least a polymer (A) and a polymer (B), in which the polymer (A) is a polymer that includes one or more monomers selected from the group consisting of acrylonitrile, vinyl halides, and vinylidene halides, the polymer (B) is a polymer that is soluble in benzyl alcohol, and the resin composition contains the polymer (A) in an amount of 70 parts by mass or more and 92.5 parts by mass or less, and the polymer (B) in an amount of 7.5 parts by mass or more and 30 parts by mass or less, where a total amount of the polymer (A) and the polymer (B) is 100 parts by mass. Provided are synthetic fibers that are easily dyeable with an acidic dye without requiring use of a special facility or heating to high temperatures.