Decomposition of NO2 (200ppm) in N-2 or air by 172-nm Xe-2 excimer lamp was studied at 1 atm. The NO2 conversion in N-2 was 99%, and the formation ratios of N-2, O-2, NO, and N2O were 47, 98, 0, and 2%, respectively, after 30 min irradiation. The NO2 in air (5-20% O-2) could be completely converted to N2O5 and HNO3 due to reactions by O-3 and H2O after only 1.0-1.5 min irradiation. The present results give a new simple photochemical aftertreatment technique of NO2 in air without using any catalysts.