A simple and miniaturized flow-through gas absorbing unit (GAU) using a small volume of an absorbing solution (0.5 mL) was developed and applied to the determination of nitrogen dioxide (NO2) in the atmosphere. The measuring method was coupled with a microflow injection analysis system (mu FIA) for a small sample size consumption. The absorption efficiency of NO2 in the GAU was improved by packing hydrophobic poly(tetrafluoroethylene) (PTFE) particles in the unit, and by using a triethanolamine aqueous solution (TEA, 2 g/L) as an absorbing solution: absorption efficiency was increased up to more than 95%. The absorbed NO2 was converted to NO3- and NO2-. The molar ratio was ascertained to be 1:1 by measuring the concentrations of NO2- and NO3- in the absorbing solution using mu FIA. Nitrogen dioxide at concentrations above 10 ppb in the atmosphere could be determined by the proposed method.