To provide a polyester-based flat-textured yarn showing less unevenness of dyeing property, soft, without having a crimp, and excellent in gloss and lightweight feeling, and also obtain the flat-textured yarn industrially.

This flat-textured yarn consisting of a polyester multifilament having ≤7% dry heat shrinkage at 180°C for 30 min and ≤50 CN/dtex Young's modulus is obtained by performing a cold stretch of the polyester multifilament having 0.02-0.080 birefringence Δn and then while performing 6-19% thermal relaxation, bringing the yarn in contact with a heating body.