Simultaneous visible spectrophotometry of interstellar object 3I/ATLAS with Seimei/TriCCS

3I/ATLAS, also known as C/2025 N$_1$ (ATLAS), is the third interstellar object (ISO) discovered in July 2025. ISOs are particularly interesting because characterizing their physical properties helps us understand and test our knowledge of Solar System formation. Several quick response observations of 3I/ATLAS were performed during the first few days after the discovery, and various results, such as reflectance spectra, have been reported. We performed simultaneous visible spectrophotometry of 3I/ATLAS from data taken using the TriColor CMOS Camera and Spectrograph (TriCCS) on the Seimei 3.8 m telescope. The Seimei/TriCCS observations of 3I/ALTAS were obtained in the $g$, $r$, $i$, and $z$ bands in the Pan-STARRS system on UTC July 15, 2025. Our lightcurves show no significant variations during the 2.3 h observation, which is in good agreement with previous studies. Visible color indices of 3I/ATLAS, $g-r=0.603\pm0.031$, $r-i=0.210\pm0.031$, $i-z=0.117\pm0.046$, and $r-z=0.327\pm0.035$ suggest it has a red surface similar to, or slightly redder than, that of D-type asteroids. Continuous observations of 3I/ATLAS before and after its perihelion passage in October 2025 are desired to investigate its physical properties.