Field observations of wintertime photochemical pollution have been conducted in order to investigate the cause of severe NO₂ pollution that often occurs in early winter in the Kanto Plain, Japan. Intensive measurements of secondary photochemical products were carried out on November 26 and 27 and December 6 and 7, 1991, when high concentrations of NO₂ (>90 ppb) were observed. Sampling sites were located in the Metropolitan Tokyo area, as well as four sites at inland of the Kanto Plain, and a site at the top of Mt. Tsukuba (800 m height, 50 km northeast of Tokyo). During the intensive measurement period, daily maximum concentrations of peroxyacetylnitrate (PAN) were 3.9-11.7 ppb on the coast at Tokyo Bay; diurnal variation was similar to that of [PO (=NO₂+0₃) NO₂^Prime (primarily emitted NO₂)]. The diurnal variation of total inorganic nitrates concentrations showed a daytime maximum except on December 7; it reached an extremely high value (59 μg m^-3) on December 6. Acetaldehyde/CO ratio also showed remarkable daytime maxima on December 6 and 7. The behavior of photochemical products indicates that the NO_x-hydrocarbons system photochemical reactions were dominant during this period. In addition, shipboard measurements in Tokyo Bay suggested that PO is predominantly in the form of O₃ above Tokyo Bay, which is contrary to land-based measurements. The results of this study further suggest that the photochemical formation of nitrate is one of the causes of suspended particulate matter (SPM) pollution, which is often accompanied by high concentrations of NO₂.