  We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of a protoplanetary disk around the T Tauri star Sz~84 and analyses of the
structures of the inner cavity in the central region of the dust disk. Sz~84's
spectral energy distribution (SED) has been known to exhibit negligible
infrared excess at $\lambda \lesssim$10~$\mu$m due to the disk's cavity
structure. Analyses of the observed visibilities of dust continuum at 1.3~mm
and the SED indicate that the size of the cavity in the disk of large
(millimeter size) dust grains is 8~au in radius and that in the disk of small
(sub-micron size) dust grains is 60~au in radius. Furthermore, from the SED
analyses, we estimate that the upper limit mass of small dust grains at
$r<$60~au is less than $\sim$10$^{-3}$~$M_{\rm earth}$, which is
$\lesssim$0.01~\% of the total (small~$+$~large) dust mass at $r<$60~au. These
results suggest that large dust grains are dominant at $r<$60~au, implying that
dust grains efficiently grow with less efficient fragmentation in this region,
potentially due to weak turbulence and/or stickier dust grains. The balance of
grain growth and dust fragmentation is an important factor for determining the
size of large dust grains in protoplanetary disks, and thus Sz~84 could serve
as a good testbed for investigations of grain growth in such disks.
