  We report the detection of an excess in dust continuum emission at 233~GHz
(1.3~mm in wavelength) in the protoplanetary disk around TW~Hya revealed
through high-sensitivity observations at $\sim$3~au resolution with the Atacama
Large Millimeter/submillimeter Array (ALMA). The sensitivity of the 233~GHz
image has been improved by a factor of 3 with regard to that of our previous
cycle 3 observations. The overall structure is mostly axisymmetric, and there
are apparent gaps at 25 and 41 au as previously reported. The most remarkable
new finding is a few au-scale excess emission in the south-west part of the
protoplanetary disk. The excess emission is located at 52 au from the disk
center and is 1.5 times brighter than the surrounding protoplanetary disk at a
significance of 12$\sigma$. We performed a visibility fitting to the extracted
emission after subtracting the axisymmetric protoplanetary disk emission and
found that the inferred size and the total flux density of the excess emission
are 4.4$\times$1.0~au and 250~$\mu$Jy, respectively. The dust mass of the
excess emission corresponds to 0.03~$M_\oplus$ if a dust temperature of 18~K is
assumed. Since the excess emission can also be marginally identified in the
Band 7 image at almost the same position, the feature is unlikely to be a
background source. The excess emission can be explained by a dust clump
accumulated in a small elongated vortex or a massive circumplanetary disk
around a Neptune mass forming-planet.
