Wind tunnel investigations of the air change rates and the mean wind speeds in an atrium lobby with a moval roof were carried out. The probabilities of exceedance of a particular air change rate are evaluated under the opening and closing conditions of the movable roof.As the results, the air change rate of the lobby is much more than the outdoor air required for air-conditioning the lobby space, even in the case of the closing condition of the movable roof. On the other hand, the mean wind speeds near the openings becomes so large that they may disturb the air velocities of air-conditioning. Consequently, the necessity of some barrier for reducing the mean wind speeds in the lobby is concluded.