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We report on the high-precision astrometric observations of maser sourcesaround the Galactic Center in the SiO J=1--0 v=1 and 2 lines with the VLBAduring 2001 -- 2004. With phase-referencing interferometry referred to theradio continuum source Sgr A*, accurate positions of masers were obtained forthree detected objects: IRS 10 EE (7 epochs), IRS 15NE (2 epochs), and SiO 6(only 1 epoch). Because circumstellar masers of these objects were resolvedinto several components, proper motions for the maser sources were derived withseveral different methods. Combining our VLBA results with those of theprevious VLA observations, we obtained the IRS 10EE proper motion of 76+-3 kms^{-1} (at 8 kpc) to the south relative to Sgr A*. Almost null proper motion ofthis star in the east--west direction results in a net transverse motion of theinfrared reference frame of about 30+-9 km s^{-1} to the west relative to SgrA*. The proper-motion data also suggests that IRS 10EE is an astrometric binarywith an unseen massive companion.

A Measurement of Proper Motions of SiO Maser Sources in the Galactic Center with the VLBA