Ultrafine fibers and liquid fiber dispersion

An ultrafine fiber having a fiber diameter (D) of 100 to 5000 nm, a ratio (L/D) of a fiber length (L) to the fiber diameter (D) of 3000 to 6000, and a carboxyl terminal group amount of 40 eq/ton or more,

• wherein at least a part of a surface layer of the ultrafine fiber is formed of polyester, when the ultrafine fiber is dispersed in an aqueous medium to obtain a fiber dispersion liquid having a solid content concentration of 0.01 to 10% by weight, a dispersion index measured by the following method is 20 or less,
• method of measuring the dispersion index:
• a fiber dispersion liquid is prepared such that the solid content concentration is 0.01% by weight with respect to a total amount of the fiber dispersion liquid, an image of the obtained fiber dispersion liquid at a magnification of 50 times is captured with a microscope under transmitted illumination, the image is converted into a monochrome image by using an image processing software, then a luminance histogram is formed with 256 grades, and a standard deviation obtained from the luminance histogram is used as the dispersion index.