  Astrometric detection of a stellar wobble on the plane of the sky will
provide us a next breakthrough in searching extrasolar planets. The Space
Interferometry Mission (SIM) is expected to achieve a high-precision astrometry
as accurate as 1 micro-as, which is precise enough to discover a new-born
Jupiter mass planet around a pre-main-sequence (PMS) star in the Taurus-Auriga
star forming region. PMS stars, however, have circum-stellar disks that may be
obstacles to the precise measurement of the stellar position. We present
results on disk influences to the stellar wobble. The density waves excited by
a planet move both of the disk's mass center and the photo-center. The motion
of the disk mass center induces an additional wobble of the stellar position,
and the motion of the disk photo-center causes a contamination in the
measurement of the stellar position. We show that the additional stellar motion
dynamically caused by the disk's gravity is always negligible, but that the
contamination of the disk light can interfere with the precise measurement of
the stellar position, if the planet's mass is smaller than ~10 Jupiter mass.
The motion of the disk photo-center is sensitive to a slight change in the wave
pattern and the disk properties. Measurements by interferometers are generally
insensitive to extended sources such as disks. Because of this property SIM
will not suffer significant contaminations of the disk light, even if the
planet's mass is as small as 1 Jupiter mass.
