To provide a polyester monofilament for a screen gauze excellent in printing accuracy since having a good dimensional stability and extremely few defects, and especially suitable for a high mesh screen gauze.

This polyester monofilament for the screen gauze has a sheath core type conjugate cross sectional surface wherein, the core component of a polyethylene naphthalate is obtained by copolymerizing 1-5 mol% isophthalic acid component based on the naphthalenedicarboxylic acid, and the sheath component is a polyethylene terephthalate, and also has (80/20) to (60/40) weight ratio of the core component/the sheath component.