PURPOSE: To obtain the titled yarn having improved passing properties through process, by subjecting phosphorus-containing polyester yarn containing a specific amount of a specified phosphorus compound to high-temperature wet heat treatment so that its intrinsic viscosity is reduced to less than a given value.

CONSTITUTION: A phosphorus compound shown by the formula (n is 3W8 integer) having ≥96% purity is added to a reaction system in a period from the end of the first-stage reaction to obtain a glycol ester of a dicarboxylic acid consisting essentially of terephthalic acid and/or its low polymer to the end of the second-stage reaction to obtain a polyester having a repeating unit comprising ≥85% ethylene terephthalate by polycondensation. The polyester containing 0.5W 1.5mol% phosphorus atom based on total acid component is subjected to melt spinning, to give phosphorus-containing polyester yarn having 0.38W0.45 intrinsic viscosity and 80μeq/g acidic end group concentration, which is heat-treated in the presence of water at ≥110°C, to reduce its intrinsic viscosity into ≤0.36.