Fugacity of $CO_2$ ($fCO_2$), temperature, salinity, nutrients, and chlorophyll-a were measured in the surface waters of southwestern East Sea/Japan Sea in July 2005. Surface waters were divided into three waters based on hydrographic characteristics: the water with moderate sea surface temperature (SST) and high sea surface salinity (SSS) located east of the front (East water); the water with high SST and moderate SSS located west of the front (West water); and the water with low SST and SSS located in the middle part of the study area (Middle water). High $fCO_2$ larger than $420{mu}atm$ were found in the West water. In the Middle water, $CO_2$ was undersaturated with respect to the atmosphere, with values between 246 and $380{mu}atm$. Moderate $fCO_2$ values ranging from 370 to $420{mu}atm$ were observed in the East water. For the East and West waters, estimates of temperature dependency of $fCO_2$ (12.6 and $15.1{mu}atm;^{circ}C^{-1}$, respectively) were rather similar to a theoretical value, indicating that SST is likely to be a major factor controlling the surface $fCO_2$ distribution in these two regions. In the Middle water, however, the estimated temperature dependence was somewhat lower than the theoretical value, and relatively high concentrations of surface chlorophyll-a coincided with the low surface $fCO_2$, implying that biological uptake may considerably affect the $fCO_2$ distribution. The net sea-to-air $CO_2$ flux of the study area was estimated to be $0.30{pm}4.81mmol;m^{-2};day^{-1}$ in summer, 2005.