<p>In order to estimate the atmospheric O<sub>2</sub> consumption in a megacity, continuous observations of atmospheric O<sub>2</sub> and CO<sub>2</sub> concentrations and of CO<sub>2</sub> flux have been carried out simultaneously at the Yoyogi (YYG) site in middle of Tokyo, Japan since March 2016. An average O<sub>2</sub> : CO<sub>2</sub> exchange ratio for net turbulent O<sub>2</sub> and CO<sub>2</sub> fluxes (OR<sub>F</sub>) between the urban area and the overlying atmosphere was obtained based on an aerodynamic method using the observed O<sub>2</sub> and CO<sub>2</sub> concentrations. The yearly mean OR<sub>F</sub> was found to be 1.62, falling within the range of the average OR values of liquid and gas fuels. Seasonally different diurnal OR<sub>F</sub> cycles at YYG indicated that the consumption of gas fuels was larger in the winter than that in the summer, especially in the morning and late in the evening. By using the OR<sub>F</sub> and CO<sub>2</sub> flux values, the annual mean O<sub>2</sub> consumption rate was estimated to be −16.3 μmol m<sup>−2</sup> s<sup>−1</sup>, which is more than 350 times larger than the global mean atmospheric O<sub>2</sub> consumption rate (about −4 μmol yr<sup>−1</sup>), implying that our life in a megacity is far from sustainable from a viewpoint of the conservation of atmospheric O<sub>2</sub>.</p>