  In this paper, we study the astrometric properties of gravitational
microlensing events caused by binary lenses. By investigating the centroid
shifts for various types of binary-lens events, we find that the deviations of
the centroid shift trajectories from the elliptical ones of single-lens events
are characterized by distortions, twistings, and big jumps. We study the
conditions of binary-lens system configurations and source star trajectories
for individual types of deviations. We find dramatic differences in the
astrometric centroid shifts for binary-lens microlensing events that would be
degenerate had their parameters been determined photometrically. Therefore,
when additional astrometric observations of a binary-lens event are available,
one can resolve the ambiguity of the binary-lens fit, and uniquely determine
the binary-lens parameters.
