To provide a flame-retardant polyester which has an excellent color tone, hardly develops deposits on a spinneret, even when the polyester is continu ously melt-spun through the spinneret for a long period, and has excellent moldability, and to provide a fiber using the flame-resistant polyester.

This flame-retardant polyester in which at least 80 mol.% of repeating units constituting the polyester are ethylene terephthalate units, and which is copolymerized with a phosphorus compound A represented by a specific structure in an amount of 0.3 to 1.5 wt.% as phosphorus on the basis of the weight of the polyester is characterized by using a titanium/phosphorus reaction product obtained by thermally reacting a titanium compound represented by a specific structure with a phosphorus compound B represented by formula (III) (see the specification) in a phosphorus element (P)/titanium element (Ti) of 1 to 4 in ethylene glycol and controlling the content of the titanium in the polyester to 2 to 40 mmol.%. And the fiber comprises the flame-resistant modified polyester.