To obtain the subject elastic fiber capable of providing a fabric excellent in dyeability, color fastness and thermoformability and capable of manifesting a good appearance, shape retaining properties and elastic recovery properties by adding a specific amount of a polyvinylpyrrolidone having a prescribed number-average molecular weight to a specified polyurethane polymer.

This elastic fiber is obtained by adding (B) 0.1-15 pts.wt. polyvinylpyrrolidone having 5,000-500,000 number-average molecular weight to (A) 100 pts.wt. polyurethane polymer prepared by reacting a polymer diol having 500-5,000 number-average molecular weight and hydroxyl groups at both ends with an organic diisocyanate and a chain extender having polyfunctional active hydrogen atoms and further preferably adding (C) 1-15 pts.wt. thermoplastic urethane resin which is a reactional product of a mixture of (i) a low-molecular diol having a 2-10C straight-chain or branched alkylene group or hydrogen groups at both ends of a bivalent alicyclic hydrocarbon group with (ii) a polymeric diol having 400-3,000 number-average molecular weight in 1-99 molar ratio of (i)/(ii) with an organic diisocyanate and having 10,000-40,000 average molecular weight thereto.