Air tightness and energy efficiency of residential buildings have been promoted in recent years, which has been a contribution to improvement of energy saving and comfortableness in the buildings. On the other hand, the problems, such as the arising of the indoor humidity by the lack of amount of ventilation, the appearing of the dew condensation, the increasing of the house dust and so on, are confirmed. The spore of the mold or a corpse and feces of the mite are included in house dust, which cause allergy, asthma, the atopy like microbial contamination. In particular, the possibility of having allergic reaction to house dust is particularly high for children whose breathing zone is near the floor. This is one of the reasons why allergic reaction symptom is increasing for them. The aim of this study is to develop a ventilation system which can remove the house dust efficiently. Numerical simulation based on CFD was performed to study the flow and diffusion fields affected by different locations and shapes of outlets. On the time of the simulation, the volume-average concentration of house dust within breathing zone increase once before it gradually decreases. From the data we can also conclude that slit exhaust ventilation system produces less house dust in comparison to ceiling exhaust.