A wet-degradable fiber having a covering resin layer is described. The moisture permeability (R) of a resin forming the covering layer is at least 2.5 g/m 2 ·24 hours as determined at a relative humidity difference of 0% and 90% with respect to a thickness of 0.1 mm. The strength wet reduction ratio (AS) of the fiber is within a range satisfying requirements represented by the following formulae: and wherein ΔS 1 represents the strength reduction ratio after two hours' immersion in water at 20°C and AS 2 represents the strength reduction ratio after fifty hours' immersion in water at 20°C.