  The uncertainty of the lens mass can be substantially reduced if it is
determined from the lens proper motion obtained from astrometric measurements
of the source image centroid shifts, $\vdelta\vtheta_c$, by using high
precision interferometers. However, for the complete resolution of the lens
parameter degeneracy it is required to determine the lens parallax by measuring
the parallax-induced deviations in the centroid shifts trajectory, $\vDelta
\vdelta \vtheta_c$. In this paper, we investigate the detectabilities of
$\vdelta\vtheta_c$ and $\vDelta\vdelt\vtheta_c$ by determining the
distributions of the maximum centroid shifts and the average maximum deviations
expected for different types of Galactic microlensing events caused by various
masses. From this investigation, we find that as long as sources are bright
enough for astrometric observations it is expected that $\vdeltavxtheta_c$ for
most events caused by masses greater than 0.1 M_\odot regardless of the event
types can be easily detected from observations by using not only the Space
Interfeormetry Mission (SIM, with a detection threshold ~ 3 \mu-as) but also
the ground-based interferometers (with a threshold ~30 \mu-as). However,
detection of $\vDelta\vdelta\vtheta_c$ from ground-based observations will be
difficult for nearly all Galactic bulge self-lensing events, and will be
restricted only for small fractions of disk-bulge and halo-LMC events, for
which the deviations are relatively large. From observations by using the SIM,
on the other hand, detecting $\vDelta\vdelta\vtheta_c$ will be possible for
majority of disk-bulge and halo-LMC events and even for some fraction of bulge
self-lensing events. For the complete resolution of the lens parameter
degeneracy, therefore, SIM observations (or equivalent) will be essential.
