  We report $0.14"$ resolution observations of the dust continuum at band 7,
and the CO(3--2) and HCO$^{+}$(4--3) line emissions toward the transitional
disk around Sz 91 with Atacama Large Millimeter/submillimeter Array (ALMA). The
dust disk appears to be an axisymmetric ring, peaking a radius of $\sim$95~au
from a Gaussian fit. The Gaussian fit widths of the dust ring are 24.6 and
23.7~au for the major and the minor axes, respectively, indicating that the
dust ring is not geometrically thin. The gas disk extends out to $\sim$320~au
and is also detected in the inner hole of the dust ring. A twin-line pattern is
found in the channel maps of CO, which can be interpreted as the emission from
the front and rear of the flared gas disk. We perform the radiative transfer
calculations using RADMC-3D, to check whether the twin-line pattern can be
reproduced under the assumption that the flared gas disk has a power-law form
for the column density and $T_\mathrm{gas}=T_\mathrm{dust}$. The thermal Monte
Carlo calculation in RADMC-3D shows that the disk temperature has a gradient
along the vertical direction beyond the dust ring, as it blocks the stellar
radiation, and thus the twin-line pattern can be naturally explained by the
flared gas disk in combination with the dust ring. In addition, no significant
depletion of the CO molecules in the cold midplane achieves a reasonable
agreement with the observed twin-line pattern. This result indicates that the
CO emission from the rear surface must be heavily absorbed in the cold
midplane.
