Abstract: A synthetic fiber which is excellent in moisture-absorbing/releasing property and exhibits high elongation and high stretch recovery, characterized in that it has moisture absorption ratios of 0.5 to 4.0% by weight in environments of 20° C.×65% RH and that of 4.5% by weight or more in environments of 30° C.×90% RH, respectively, and also has a difference between the absorption ratios in both the environments of 4.0% by weight or more. The representative examples of such fibers include polyurethane synthetic fiber and polyether synthetic fiber. The synthetic fiber maintains a high strength at break of an elastic fiber component thereof also in the state of having absorbed moisture, is excellent in color fastness to rubbing, and can be used for manufacturing a stretch fiber fabric product excellent in comfort through blending with another fiber material.