To provide a method for producing an undrawn polyester yarn having high elongation in high efficiency while suppressing the orientation in spinning and improving the process stability in spinning.

The undrawn polyester yarn having a birefringence of 0.005-0.070 is produced by the melt-spinning of a polyester composition obtained by incorporating a polyester with 0.3-5.0 wt.% polymer incompatible with the polyester. The polyester is produced by the polycondensation in the presence of a titanium alkoxide or a reaction product of a titanium alkoxide with an aromatic polybasic carboxylic acid (or its anhydride) and a specific phosphonoacetic acid derivative and having a Ti/P ratio falling within a specific range.