  Continuing work initiated in an earlier publication (Abe, ApJ, 725 (2010)
787), we study the gravitational microlensing effects of the Ellis wormhole in
the weak-field limit. First, we find a suitable coordinate transformation, such
that the lens equation and analytic expressions of the lensed image positions
can become much simpler than the previous ones. Second, we prove that two
images always appear for the weak-field lens by the Ellis wormhole. By using
these analytic results, we discuss astrometric image centroid displacements due
to gravitational microlensing by the Ellis wormhole. The astrometric image
centroid trajectory by the Ellis wormhole is different from the standard one by
a spherical lensing object that is expressed by the Schwarzschild metric. The
anomalous shift of the image centroid by the Ellis wormhole lens is smaller
than that by the Schwarzschild lens, provided that the impact parameter and the
Einstein ring radius are the same. Therefore, the lensed image centroid by the
Ellis wormhole moves slower. Such a difference, though it is very small, will
be in principle applicable for detecting or constraining the Ellis wormhole by
using future high-precision astrometry observations. In particular, the image
centroid position gives us an additional information, so that the parameter
degeneracy existing in photometric microlensing can be partially broken. The
anomalous shift reaches the order of a few micro arcsec. if our galaxy hosts a
wormhole with throat radius larger than $10^5$ km. When the source moves
tangentially to the Einstein ring for instance, the maximum position shift of
the image centroid by the Ellis wormhole is 0.18 normalized by the Einstein
ring radius. For the same source trajectory, the maximum difference between the
centroid displacement by the Ellis wormhole lens and that by the Schwarzschild
one is -0.16 in the units of the Einstein radius.
