The nitrogen isotopic composition of nitrogen oxide (NOx) is useful for estimating its sources and sinks. Several methods have been developed to convert atmospheric nitric oxide (NO) and/or nitrogen dioxide (NO2) to nitrites and/or nitrates for collection. However, the collection efficiency and blanks are poorly evaluated for many collection methods. Here, we present a method for collecting ambient NOx (NO and NO2 simultaneously) with over 90 % efficiency collection of NOx and low blank (approximately 0.5 µM) using a 3 wt% hydrogen peroxide (H2O2) and 0.5 M sodium hydride (NaOH) solution. The 1σ uncertainty of the nitrogen isotopic composition was {plus minus} 1.0 ‰. The advantages of this method include its portability, simplicity, and the ability to collect the required amount of sample to analyze the nitrogen isotopic composition of ambient NOx in a short period of time. Using this method, we observed the nitrogen isotopic compositions of NOx (δ15N(NOx)) at the Tsukuba and Yoyogi sites in Japan. The averaged value and standard deviation (1σ) in the Yoyogi site was (−2.7 {plus minus} 1.8) ‰ and in the Tsukuba site was (−1.7 {plus minus} 0.9) ‰ during the sampling period. The main NOx source appears to be the vehicle exhaust in the two sites. If the developed method is used by many researchers in the future, the database of δ15N(NOx) values will be enriched and a deeper understanding of NOx behavior will be gained.