Recently, there have been many office buildings with atriums. It is important to make clear the contribution of atriums to infiltration and airflow within the buildings caused by stack effect. In this paper, computer simulation of the air leakage is conducted for seven types of atrium. As the results, there is the different feature in each atrium type and infiltration rate can be 0.7〜1.5 times of the rate in the standard atrium type. And this paper discusses the other two problems. One is the problem of the effects of frequently opened doors, and another is natural ventilation of the hot space at the top of an atrium in summer.