To obtain a self-elongation polyester filament yarn providing combined filament yarns giving bulkiness and a soft feeling without giving fluffy or slimy feeling to woven and knitted cloth by subjecting a specific highly oriented undrawn polyester filament yarn to contact heat-treating on a heater under a specific condition.

The objective self-elongation polyester filament yarn having a shrinkage satisfying conditions expressed by the formulae: SHD--SHW≤0 and SHW≤8 [SHW is boiling water shrinkage (%); SHD is dry heat shrinkage at 160°C after treating in boiling water (%)] and forming streak-like grooves of each 0.1-3μm breadth and ≥5μm length in the fiber axis direction at a ≤5μm adjacent interval on the fiber surface in weight losing by an alkali is obtained by subjecting a highly oriented undrawn polyester filament yarn 1 comprising ethylene terephthalate as a main repeating unit and containing 0.1-10wt.% of a metallic salt of an organic sulfonic acid of the formula R-SO3M (R is a 3-30C alky or a 7-40C aryl; M is an alkali metal) and having birefringence (n) of 0.03-0.08 to contact heat-treating on a heater 3 at glass transition point-130°C under a tension of 0.02-0.12g/d.