An object of the present invention is to provide polyurethane elastic fibers that, when dyed with acidic dyes, exhibit a deep dyeing property and high fastness equivalent to those of polyamide fibers used as a companion material in mixed knitted and woven fabrics, while also exhibiting excellent spinning stability. The inventive polyurethane elastic fibers comprise 0.2 wt%-10 wt% of a polymer with a maleimide structure comprising an isobutylene unit represented by the following formula (I) and a maleimide unit represented by the following formula (II) (wherein R 1 represents a C 2-6 straight-chain or branched alkylene group, and R 2 and R 3 may be the same or different and each represents a C 1-6 straight-chain or branched alkyl group), have a Blue L value of 20-50 after laundering according to JIS L0844 A2 for dyeing of a single-loop tube-knit fabric with a specific blue acidic dye, and have a staining degree of grade 3.5 or greater on nylon fabrics.