Measurements of the heating condition of a steel beam installed beneath a ceiling and exposed to a localized fire source are made on a real-scale experiment. The data of thermal response obtained from the experiments are compared with previous small-scale experiments. The effects of the smoke layer which influences upon the heating condition of the beam are investigated through the smoke experiments setting the smoke protection soffits to the same experimental equipment. FDM-based calculation is demonstrated using the average temperature of the smoke layer for the boundary conditions to predict the thermal response of the beam. Applicability of the approximated temperature of the smoke layer is examined by comparing the numerical results of the temperature with those obtained through the experiment.