In order to investigate the effect of buoyancy on soot formation in gas-jet diffusion flame, we conducted one set of experiments with the High-Temperature Air Combustion Technology (HiCOT) system and another set under partial gravity conditions. Ethylene ($C_2H_4$) was used as fuel, and soot volume fractions for the flame were observed as shadow graph images with backlight. In the experiment with the HiCOT, the oxygen concentrations were $O_2$ = 15 %, 17 %, and 23 %, with constant flame temperature and surrounding air temperatures of 1100 K, 900 K, and 300 K, respectively. We found that the soot volume fraction in the flames increased with the increase of the oxidizer temperature. In the partial gravity experiment meant to identify the buoyant effect, the results showed that the soot volume fraction depended on the gravity level. These results imply that soot formation in a gas-jet diffusion flame with the HiCOT is strongly affected by the buoyant flow due to oxidizer flow into the soot formation field.