A fine cellulose fiber in which when the fine cellulose fiber is formed into a dispersion, the dispersion has very high light transmittance and viscosity, and a method for producing the fine cellulose fiber. The fine cellulose fiber has a fiber width of 1 to 200 nm. A part of hydroxy groups of the cellulose fiber are substituted with a predetermine functional group to introduce an ester of phosphorus-oxo acid, and the introduction amount of the functional group is more than 2.0 mmol per 1 g of the cellulose fiber. For producing the fine cellulose fiber, to cellulose fiber is added a solution having a pH of less than 3.0 and including an additive (A) containing at least one of a phosphorus-oxo acid and a phosphorus-oxo acid metal salt and an additive (B) containing at least one of urea and a urea derivative, and the mixture is heated to perform fibrillation.