Flame retardant treating agents according to the invention comprise a phosphorus-based compound represented by the following formula (4): ##STR1##

Flame retardant treated fibers can be obtained by immersion of polyester fiber woven fabrics and the like in a treatment solution containing the flame retardant treating agents and accomplishing heat treatment at a prescribed temperature. It is thereby possible to impart adequate flame retardance to polymer materials such as fibers and thermoplastic polymers even when used in small amounts, and provide flame retardant treating agents that contain no halogen compounds.