In some industrial buildings, ventilation system continues to be operated even after the break of fire in order to keep the necessary pressure differences. In such situations, supplied air at the ceiling creates a downward plume. The plume entrains smoke in the upper layer and move the smoke particles downward. As a result, lower air layer is contaminated by smoke. In this paper, model scale experiments were carried out to investigate the properties of downward plume. A smoke layer was created in the upper part of a model room. Fresh air was supplied downward via an outlet equipped at the ceiling. The velocity and temperature profiles and the mass flow rate of downward plume was measured by direct measurement method and gas analysis method. The mass flow rate was formulated by Zukoski’s plume formula and compared with experimental measurements. To evaluate the degree of contamination of the lower layer, the yield ratio of downward plume to lower layer was investigated with the Froude number at the bottom of smoke layer. The yield ratio was around 0.8.