If a ceiling plenum in a house can be made into the route of airflow for ventilation without using duct, it is possible to design that it be made still higher the ceiling height of a sitting room, and that it frees a floor plan. As a result of the experiment using model made to the same measure of ceiling plenum in a detached house constructed of box frame type reinforced concrete, this report presents the effect of decrease of total pressure loss in ceiling plenum chamber by processing opening for the purpose of decreasing resistance of airflow.