Observations were made of time variations of carbon dioxide in seawater, pCO 2 , and in the atmosphere, PCO 2 , in the Seto Inland Sea of Japan. The pCO 2 data showed well defined diurnal variation; high values at nighttime and low values during daylight hours. The pCO 2 correlated negatively with dissolved oxygen. These results denote that the diurnal variation of pCO 2 is associated with effects of photoplankton's activity in seawater. The pCO 2 measured in the Seto Inland Sea showed higher values than the PCO 2 during June to November, denoting transport of carbon dioxide from the sea surface to the atmosphere, and lower values during December to May, denoting transport of carbon dioxide from the atmosphere to the sea surface. The exchange rates of carbon dioxide were calculated using working formula given by Andrié et al.