We have developed a real-time holographic interferometry system with high-speed camera in order to visualize two dimensional, time-dependent temperature distributions in gas and solid phases simultaneously during the combustion of transparent thermoplastic material. The ultimate goal of this study is to contribute to upgrade the flame spread modeling at which the complex physical processes (e.g., fuel regression, bubble formation, motion inside the molten layer) have been mostly ignored, although those effect are not so well-known and not well-studied. As first step, thermal response of thermoplastic material subjected to the disturbance in gas phase was investigated. Thick transparent PMMA slab was used as sample specimen and the thermal status in gas, molten and solid phases over spreading flame downwardly was examined. Whole optical set was carefully arranged and tuned to obtain satisfactory clear interference fringes appeared in gas and solid phases during the combustion event. Following the disturbance introduced in the system, the time change of refractive index in gas, molten and solid phases and the corresponding temperature distributions was recorded via a high-speed camera. Response delay time was carefully analyzed to discuss the potential role of the liquid phase on the burning character.