To provide a polyester combined filament yarn using thick and thin ultrafine fiber as a sheath yarn, excellent in natural unevenness and further having soft spun like touch feeling and high-grade feeling and to provide a method for producing the blended yarn.

This polyester combined filament yarn is obtained by mutually interlacing a polyester filament yarn of a highly oriented undrawn yarn A having 1-5 dtex single fiber filament denier used as a core yarn with a thick and thin polyester highly oriented undrawn yarn B having ≤1 dtex single fiber filament denier and thick and thin difference used as a sheath yarn each by loop and slackness. The polyester combined filament yarn has the distribution of yarn unevenness exhibiting 100-500/m total numbers of loop and slackness, 3-15% Uster irregularity U(%) and 6-25 cm peak value of spectrogram.