The O2/N2 ratios were determined of air samples collected at Hateruma Island (lat 24¿3'N, long 123¿49'E) from July 1997 to December 2001, and at Cape Ochi-ishi (lat 43¿10'N, long 145¿30'E) from December 1998 to December 2001. The O2/N2 ratios, based on gas-chromatographic analysis, clearly showed decreasing trends and seasonal variations, increasing in spring and summer, and decreasing in autumn and winter. The amplitudes of the seasonal O2/N2 cycles for Ochi-ishi were twice as large as those for Hateruma. The average rates of decrease were 18.4 ¿ 1.4 per meg yr-1 over a 4-year period (1998--2001) for Hateruma, and 19.2 ¿ 1.6 per meg yr-1 over a 3-year period (1999--2001) for Ochi-ishi.