The flame retardant treating agent of the invention comprises a phosphorus-based compound represented by the following formula (4), for example. ##STR1##

According to the invention, a polyester fiber woven fabric is immersed in a treatment solution containing the flame retardant treating agent and is heat treated at a prescribed temperature in order to fix the flame retardant treating agent to the polyester fiber woven fabric to obtain flame retardant treated fiber. As a result, it is possible to maintain adequate fiber cleaning durability and achieve a sufficient flame retardant property even by post-treatment, without using halogen-based compounds.