  To reveal the structures of a transition disk around a young stellar object
in Lupus, Sz 91, we have performed aperture synthesis 345 GHz continuum and
CO(3--2) observations with the Submillimeter Array ($\sim1\arcsec$--3$\arcsec$
resolution), and high-resolution imaging of polarized intensity at the
$K_s$-band by using the HiCIAO instrument on the Subaru Telescope ($0\farcs25$
resolution). Our observations successfully resolved the inner and outer radii
of the dust disk to be 65 AU and 170 AU, respectively, which indicates that Sz
91 is a transition disk source with one of the largest known inner holes. The
model fitting analysis of the spectral energy distribution reveals an H$_2$
mass of $2.4\times10^{-3}$ $M_\sun$ in the cold ($T<$30 K) outer part at
$65<r<170$ AU by assuming a canonical gas-to-dust mass ratio of 100, although a
small amount ($>3\times10^{-9}$ $M_\sun$) of hot ($T\sim$180 K) dust possibly
remains inside the inner hole of the disk. The structure of the hot component
could be interpreted as either an unresolved self-luminous companion body (not
directly detected in our observations) or a narrow ring inside the inner hole.
Significant CO(3--2) emission with a velocity gradient along the major axis of
the dust disk is concentrated on the Sz 91 position, suggesting a rotating gas
disk with a radius of 420 AU. The Sz 91 disk is possibly a rare disk in an
evolutionary stage immediately after the formation of protoplanets because of
the large inner hole and the lower disk mass than other transition disks
studied thus far.
