  Accurate estimation of brightness of (3200) Phaethon up to lower phase angles
are essential for planning of the on-board camera of the DESTINY$^{+}$ mission.
We have carried out intensive observations of Phaethon in the optical
wavelength ($g$, $r$, and $i$) with the TriCCS camera on the Seimei 3.8 m
telescope in October and November, 2021. We derived the absolute magnitude
$H_\mathrm{V}$ and the slope parameter $G$ of Phaethon as
$H_\mathrm{V}=14.23\pm0.02$ and $G=0.040\pm0.008$ from multiple photometric
observations including lower phase angles down to $\sim$9$^{\circ}$ with the
$H$-$G$ model. Using the $H_\mathrm{V}$ value and the geometric albedo of
Phaethon derived in previous polarimetric studies, we estimated that the
Phaethon's diameter is within a range of 5.22 to 6.74 km, which is consistent
with radar and occultation observations. With the linear model, we derived
$H_\mathrm{V}=14.65\pm0.02$, which corresponds to a diameter range of 4.30 to
5.56 km. Our simultaneous tricolor lightcurves of Phaethon indicate that no
rotational spectral variations larger than 0.018 and 0.020 mag in the g-r and
r-i colors, possibly related to inhomogeneity of the surface material and/or
structure, are seen at the 2021 apparition.
