Provided is a superfine polyester fiber containing at least 98 wt% of a polyethylene terephthalate component, and characterized by fulfilling the following conditions: (1) the reduced viscosity (ηsp/c) is at least 0.80 dl/g； (2) the total fineness is 7-120 dtex, and the single fiber fineness is no more than 0.5 dtex； the toughness parameter (X) indicated by formula (1) is at least 2.0, the tensile strength is at least 3.5 cN/dtex, and the tensile elongation is at least 12%. X = (tensile strength ×√tensile elongation)/(total fineness × single fiber fineness) … formula (1).
