Compared to that in the open ocean, the behaviour of CO2 in coastal seas has not been studied well. In this study, based on in situ data obtained in the Seto Inland Sea, we explained the spatial distribution of pCO2, focusing on regional characteristics. In addition, we analysed the difference in the parameters contributing to the CO2 flux. pCO2 was lower in spring than in summer, suggesting the effects of seawater temperature. In summer, pCO2 in most straits was higher than that in the adjacent non-strait areas. This is suggested that high-CO2 water masses, which produced by organic matter decomposition in the lower part of the stratified area, flowed horizontally into the adjacent straits and was then transported to the surface by vertical mixing. In particular, pCO2 in Osaka Bay was the lowest during both seasons, suggesting CO2 consumption by high primary production. CO2 was absorbed in entire region in spring, while in summer, that was absorbed in the Pacific, Bungo Channel, Kii Channel and Osaka Bay, emitted in the remaining area. Regarding CO2 fluxes, pCO2 contributed the most to the direction of release or absorption, and U contributed the most to the magnitude of CO2 fluxes. Although T and S hardly contributed directly to sea-air CO2 exchange, they indirectly contributed to the direction of the CO2 fluxes.