Water-soluble flame-retardant polyester resin, resin composition containing the resin, and fiber product treated with the resin composition

A water-soluble, flame retardant polyester resin prepared by a condensation reaction or a polycondensation reaction of a dicarboxylic-acid component, a glycol component, a water-solubility imparting component and a reactive phosphorus-containing compound such that a ratio of the water-solubility imparting component in a total of the dicarboxylic-acid component and the water-solubility imparting component is in a range of 5 to 40 mol,

• wherein the water-solubility imparting component consists of at least one of a tribasic acid anhydride and a tetrabasic acid anhydride, and
• wherein the reactive phosphorus-containing compound is at least one selected from compounds represented by the following general formulas (I) (II) and (III):
• wherein, in the formula I, “R1” to “R8” respectively designate hydrogen atom or an organic group, which may be equal to or different from each other, “A” designates hydrogen atom or an organic group, which may be equal to or different from “R1” to “R8”, but at least one of “R1” to “R8” and “A” has an ester-forming functional group,
• wherein, in the formula II, “R9” and “R10” respectively designate hydrogen atom or an organic group, which may be equal to or different from each other, but at least one of “R9” and “R10” has an ester-forming functional group,
• wherein, in the formula III, “R11” to “R13” respectively designate hydrogen atom or an organic group, which may be equal to or different from each other, but at least one of “R11” to “R13” has an ester-forming functional group.