  We have developed a perturbative approach to microlensing due to an
extrasolar planetary lens. In particular, we have found analytic formulae for
triple images. We have used the formulae to investigate the astrometric
microlensing due to the extrasolar planetary lens, in expectation of dramatic
improvements in the precision of the future astrometric measurements. For a
weak lensing case, we have shown how the maximum angular size and the typical
time scale of the anomalous shift of the lightcentroid are dependent on the
mass ratio and angular separation between the star and the planet.
