Nitrogen oxide gas (NOx), consisting of nitrogen monoxide (NO) and nitrogen dioxide (NO2), at a low concentration corresponding to that on roads as a result of exhaust from automobiles, was supplied for 25 days through a laboratory-scale biofilter packed with soil as a packing material. The removal efficiency of NO2 by soil was almost 100%, and the removal efficiency of NO was 60% on average and 86% at maximum. By using gamma-irradiated soil as a packing material, NO2 was completely removed mainly by adsorption onto or absorption into the packing material. However, the removal efficiency of NO in the sterilized soil was only 20%, suggesting that NO in soil was removed microbiologically under aerobic conditions.