In order to simulate air flow in the vertical laminar flow type clean room with floor opening, resistance distribution of floor opening must be considered. In this paper, new concepts of porous media model were introduced into the previous numerical model. The computed results based on the new model were compared with the experimental results. As a result, computed distributions of air velocity and deflected angle of air flow agreed with two dimensional experimental data. Further, the computer simulation technique was applied to examine the relationship between the air flow pattern and the opening ratio of the floor with installation in an actual vertical laminar flow type clean room.