To avoid an influence on measurement accuracy in a case where an observation window for a measurement sample is provided to a thermal analysis apparatus, the influence being imposed by thermal conduction through the observation window, the observation window is formed of layers of transparent members, and a gap layer is provided between the layers, to thereby reduce the thermal conduction. Gas or solid having a high heat insulation property is employed for the gap layer to further enhance a heat insulation property of the observation window. Accordingly, a change due to heating of the measurement sample is visually observed in the thermal analysis apparatus, to thereby obtain a thermal change or a physical change with higher accuracy.