A rayon fiber that does not use a halide-based flame retardant, a phosphorous flame retardant or an organic solvent, has excellent flameproofness and launderability and is biodegradable when buried in the soil at the time of disposal, and a method for manufacturing the same are provided. The flameproof rayon fiber according to the present invention contains components of silicon and magnesium, and a compound containing the components of silicon and magnesium is amorphous. This rayon fiber can be manufactured by preparing a viscose solution, adding a solution containing a silicate compound containing an alkali metal to the viscose solution so as to make a silicic compound-added viscose spinning solution containing the alkali metal, performing spinning by extruding the silicate compound-added viscose spinning solution containing the alkali metal through a spinneret into a spinbath containing a sulfuric acid, thus producing a fiber to be treated containing the silicate compound, and treating the fiber to be treated using a magnesium-containing solution in a scouring process or an aftertreatment process.