Abstract Accurate estimations of the brightness of (3200) Phaethon up to lower phase angles are essential for the planning of the on-board camera for the DESTINY+ mission. We 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 2021 October and November. We derived the absolute magnitude HV and the slope parameter G of Phaethon as HV = 14.23 ± 0.02 and G = 0.040 ± 0.008 from multiple photometric observations including lower phase angles down to ∼9° with the H–G model. Using the HV value and the geometric albedo of Phaethon derived in previous polarimetric studies, we estimated that Phaethon’s diameter is within a range of 5.22–6.74 km, which is consistent with radar and occultation observations. With the linear model, we derived HV = 14.65 ± 0.02, which corresponds to a diameter range of 4.30–5.56 km. Our simultaneous tricolor light curves 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.