A porous fence made of activated carbon fiber (ACF) has been developed to reduce high NO₂ concentration at the roadside of heavy traffic. Polluted air over the road moves with natural wind to contact with and to flow through the ACF layer, and then the ACF removes NOx from the dirty air. This idea of “energy-free” equipment for air pollution reduction is proposed and numerically evaluated. Two important parameters on the ACF fence are identified as the resistance to the air flow and the chemical reactivity for NOx removal. With these two parameters varied, the performance of double fences along the road and single fence set on the lee side of the road is investigated. Obtained results show:(1) for the double fences, large resistance of the fences to the air flow is rather preferable for effective reduction of ambient NOx concentration, because this large resistance enhances contact of polluted air over the road with the ACF fence on the upwind side, (2) for the single fence, on the other hand, small registance of the fence is better for the NOx removal, and (3) fmally, the ACF fences are shown to be able to reduce ambient NOx by more than 30% without excess energy use.