A high precision continuous measurement system has been developed for analysis of the atmospheric O-2/N-2 and Ar/N-2 ratios based on a mass spectrometry method. Sample and reference air flows through an inlet system and only a miniscule amount of each is transferred to the ion source of the mass spectrometer through thermally insulated thin fused silica capillaries. The measured O-2/N-2 and Ar/N-2 values are experimentally corrected for the effects of pressure imbalance between the sample air and reference air during their introduction into the mass spectrometer, as well as for the influence of CO2 concentration and O-2/N-2 ratio of the sample air. Standard deviations of the measured O-2/N-2 and Ar/N-2 ratios of standard air are +/-3.2 and +/-6.5 per meg, respectively, for our normal measurement time of 62 seconds. Our standard air is prepared by drying natural air and then stored in 48-L high-pressure cylinders; its O-2/N-2 and Ar/N-2 ratios are stable to within +/-1.1 and +/-5.8 per meg, respectively, over a period of 11 months. The CO2/N-2 ratio is also simultaneously measured by this system, and converted to CO2 concentration with a precision better than +/-0.3 ppm using an experimentally determined relationship. This system has been field tested in Tsukuba, Japan since February 2012. Preliminary results show clear seasonal cycles of atmospheric potential oxygen (APO = O-2 +1.1 x CO2), as well as of Ar/N-2. If we ignore the fossil fuel influence, then that part the seasonal APO cycle driven by the air-sea heat flux accounts for 23% of the observed seasonal APO cycle, as estimated from the seasonal cycle of Ar/N-2; any residuals are attributed to ocean biology and ventilation.