PURPOSE: To impart excellent oil-repellent and water-repellent properties, and soil-release(SR) properties in dry and wet conditions to a fiber by treating the synthetic or natural fiber with fluorocarbonylimino biurets.

CONSTITUTION: This synthetic or natural fiber having durable antifouling characteristics is obtained by treating the synthetic or natural fiber with carbonylimino biurets of formulas I or II [Z is a tris(carbonylimino) biuret group of formula III (R is a 4-20C aliphatic or alicyclic group); Rf is a completely fluorinated 3-20C aliphatic group; A is a divalent group of SO2NR3, CONR3, S or SO2; B is a divalent linear hydrocarbon group CnH2n; X is O, S or the like; R' is a monovalent aliphatic, alicyclic, aromatic group or the like, containing no fluorine and substituted with Ce, Br or the like; R" is a (substituted) aliphatic, alicyclic, aromatic group or the like containing (no) fluorine; (m)is 0 or 1; (p) is 1 or 2; (q) is 2 or 1;(r)is 1 or 2; δ is (2-r); and (t)is 2-100].