PURPOSE: To simply and efficiently obtain a uniform fine fiber having small fiber diameter.

CONSTITUTION: Shear force is exerted on a composite of sea-island structure formed of a resin composition consisting essentially of two or more kinds of thermoplastic resins having different recurring units (e.g. combination of a styrene-based polymer with an olefinic polymer) and having an island with ≥3 average aspect ratio to provide the objective fine fiber. The shear force can be given by passing a dispersion obtained by dispersing a film, a fiber, etc., having a sea-island structure into a solvent through a small-sized orifice by the difference in pressure of at least 200Kg/cm2 and allowing the dispersion to collide with a wall body at a high speed and rapidly decreasing the rate. The fine fiber comprising a fiber having 0.05-2μm average fiber diameter is obtained by this method.