In a large enclosure containing contaminant sources such as an underground car park, it is important to supply fresh air for the occupied zone and to remove polluted air efficiently. From the viewpoint of human health, therefore, the layouts and the ventilation rate of supply openings as well as exhaust openings should be designed properly to keep air quality favorable for health. In the previous papers, the experimental study on the age of air in an underground car park with the one-way ventilation system was presented. The detail of fresh air distribution was studied with the CFD simulation conducted in the same conditions as the experiment did. In addition, the influence of supply/exhaust air imbalance of mechanical ventilation system, and the buoyancy effect of heat from ground and vehicles were studied with the age of air obtained from CFD. In this paper, the distribution of polluted gas generated from vehicles is simulated. Two locations of pollutant source, i.e. a parking space and a driving path, are supposed, and the ventilation efficiency ig examined by CFD. In addition, in order to improve the ventilation efficiency, high velocity fan diffuser-units are supposed to be installed in the stagnant region and investigated with CFD simulation.