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We have performed high-precision astrometry of H2O maser sources in Galacticstar forming region Sharpless 269 (S269) with VERA. We have successfullydetected a trigonometric parallax of 189+/-8 micro-arcsec, corresponding to thesource distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax evermeasured, and the first one detected beyond 5 kpc. The source distance as wellas proper motions are used to constrain the outer rotation curve of the Galaxy,demonstrating that the difference of rotation velocities at the Sun and at S269(which is 13.1 kpc away from the Galaxy's center) is less than 3%. This givesthe strongest constraint on the flatness of the outer rotation curve andprovides a direct confirmation on the existence of large amount of dark matterin the Galaxy's outer disk.

Astrometry of Galactic Star Forming Region Sharpless 269 with VERA : Parallax Measurements and Constraint on Outer Rotation Curve