Investigations of air pollutants were conducted aboard a research vessel anchoring for several hours at several sites while cruising over Osaka Bay and the surrounding sea area in Japan. The concentrations of nitrogen oxides (NOx) within Osaka Bay sometimes exceeded 60 ppb (parts per billion by volume) and showed a clear diurnal variation pattern with a broad peak in the daytime. A similarity was observed between the variation patterns of NOx concentration and sea traffic on the bay. Outside the bay, NOx concentration was constant at about 13 ppb. Atmospheric monitoring from the rooftop of a skyscraper on the coast often showed high levels of NOx in air masses from Osaka Bay, seemingly affected by ship emissions. On the other hand, low NOx concentrations (< 5 ppb), which never appeared in the air masses from the urban area, were also often observed. The effects of NOx emissions from ships on inland air quality thus seem to vary widely depending on the density of sea traffic on the bay.