The present invention provides fiber and a fiber structure having a high flame-retarding property and high moisture-absorptive property which do not generate noxious gases such as hydrogen halide gas upon burning, do not elute heavy metal compounds and phosphorus compounds therefrom even when reclaimed upon discarding including a burning treatment and have an excellent processing property. The present invention discloses a highly flame-retarding and moisture-absorptive fiber, characterized in that, it comprises an organic polymer having a cross-linking structure and a salt-type carboxyl group in which at least a part of such a salt-type carboxyl group is a magnesium salt type and a saturated moisture absorption rate at 20° C. and 65% relative humidity and a limiting oxygen index are not less than 35% by weight and not less than 35, respectively, and a flame-retarding fiber structure wherein the highly flame-retarding and moisture-absorptive fiber is used in at least a part of the structure.