The flux of nitric oxide radical (NO•) during the daytime at the Seto Inland Seaatmosphere boundary was measured during the sea cruises embarked upon in September, 2013 and June, 2014. Atmospheric concentration of NO• over the Seto Inland Sea and that present in the sea surface were measured. Results showed that average atmospheric NO• concentrations measured over the Inland Sea in 2013 and 2014 were 4.4 × 10-10 and 5.2 × 10-10 atm, respectively. Also during the cruises, average wind speed of 2.0 and 3.2 m s-1 were measured in 2013 and 2014, respectively. In the surface seawater, the average NO• concentrations measured were 2.1 × 10-11 and 1.9 × 10-11 mol NO• L-1 in 2013 and 2014, respectively. Using appropriate formula, a daytime efflux of 0.22 pmol NO• m-2 s-1 from the sea to the atmosphere was estimated for the two year-study. Assuming 8 h of solar intensity per day, about 1.54 × 106 g NO• y-1 efflux from the 23,000 km2 Seto inland Sea surface was determined. The study confirmed that, during the day, sea could be an additional source of NO• to the atmosphere.