A fiber made of a polyester copolymer (B), wherein the polyester copolymer (B) comprises a dicarboxylic acid component and a glycol component, and the dicarboxylic acid component comprises 75 mol % or more of a terephthalic acid component, 1.0 mol % to 3.5 mol % of component (a) derived from a compound represented by formula (I), 2.0 mol % to 10.0 mol % of a cyclohexane dicarboxylic acid component (b), and 2.0 mol % to 8.0 mol % of an aliphatic dicarboxylic acid component (c). This can afford a polyester fiber that exhibits deep color property to cation dyes and disperse dyes under a normal pressure environment and is superior in color fastness to washing and color fastness to light and can secure good spinnability.