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 (·sp/c) is at least 0.80 dl/g； (2) the total fineness is 7-120 dtex, and the single filament fineness is no more than 0.5 dtex； and (3) 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 × ˆštensile elongation / total fineness × single filament fineness
