The use of phase change material (PCM) is considered to be an effective method for energy conservation in buildings. In the present study, the thermal behavior of PCM and the temperature fluctuation in the PCM-installed room were accessed. The main findings are summarized as follows: 1. The differences of the temperature fluctuation among the empty, furnished, and PCM-installed space were clearly showed experimentally. 2. PCM and the latent heat storage materials (A4 paper size) serving as furniture were considered as three-dimensional heat conductors and an endothermic response of the materials was induced by using the finite difference method. Numerical analysis on the room temperature, which is calculated by thermal load calculation program (LESCOM-mint), successfully demonstrated the experimental results.