This study was conducted to develop an activated carbon filter to remove NO2 and HONO in indoor air. Various design parameters such as the loading rates of metals and non-metals, precursors of the metals, and mixture ratios were investigated. The removal efficiency of NOx by generally commercial ACs was as a high as 75%, depending on diverse factors, including the specific surface area of ACs, the particle size, pore structure, carbonization temperature, and others. With the same loading rate (1.0 wt.%), the RE by K (74%) was generally higher than those by Cu (66%), Mn (60%), and AC alone (67%). However, we did not find remarkable differences for different loading rates for the same K precursor. Based on these results, we carried out a pilot experiment to determine the optimum AC to remove odorous compounds (such as acetaldehyde, ammonia, and acetic acid) as well as NO2 and HONO, and found that gaseous air pollutants in indoor air can be sufficiently and efficiently removed by an improved activated carbon filter impregnated with two types of compounds (2.5% PABA + 6% H3PO4 and 3% KOH). However, more studies, as mentioned above, should be conducted to determine whether the results of this study will apply for actual indoor conditions.