Provided by the present invention is a method for simultaneously dyeing and flame-retardant finishing a polyester-based fiber product, which is excellent regarding dyeing reproducibility, the method including immersing and heating a polyester-based fiber product in a processing bath containing a specific yellow disperse dye and a phosphoramidate represented by Formula (V):

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Provided by the present invention is a method for simultaneously dyeing and flame-retardant finishing a polyester-based fiber product, which is excellent regarding dyeing reproducibility, the method including immersing and heating a polyester-based fiber product in a processing bath containing a specific yellow disperse dye and a phosphoramidate represented by Formula (V):

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