The carbon dioxide in seawater (pCO2) was measured in the Seto Inland Sea of Japan using newly developed equilibrator instrument designed to be free from the correction for addition or extraction of the carbon dioxide from carrier gas. The temperature dependence of pCO2 was about 4.5%pCO2/°C for a single seawater sample which was processed as free from biological activity and change in total carbon dioxide content during an experiment. The decrease in pCO2 during daylight hours due to the photosynthetic fixation was about 30% of the daily mean of pCO2 during warm months and about 15% during cold months. The effect of carbon dioxide exchange between air and seawater on pCO2 was about 0.6 ppm in August and about 0.1 ppm in March. This is negligible small compared with the daily oscillation of carbon dioxide in seawater. © 1993 Journal of the Oceanographic Society of Japan.