For the purpose of analyzing the recent trend of the photochemical oxidants (Ox) in urban areas, changes in the Ox and NO_x concentrations and O₃ decrease due to the NO-titration, Titr (O₃), between two periods, 2000–2005 and 2010–2015, have been investigated, first in the major urban and surrounding regions of the Kanto district. Nocturnal Ox generally increased by 0–10 ppb between the two periods, and the changes well corresponded with the decrease in the Titr (O₃) at each site. The Ox increases, when accumulated for one day, compensated for the daytime decreases in the summertime afternoons, and increased the daily averages. As for the springtime, the nocturnal Ox increases tended to exceed the decreases in the Titr (O₃), which suggested increases in the background ozone in the spring, although its contribution could be estimated to be no more than 5 ppb. Similar analyses for the Kinki, Aichi, and Fukuoka urban regions exhibited somewhat different features from each other in changes in the Ox during the two periods. However, the springtime tendency that the nocturnal Ox increases exceeded the decreases in the Titr (O₃) was recognized in every region.