In the wet forming of ceramics, it is critical to control the cracking and deformation of the ceramic slurry during the drying process. Since these defects are carried over to the sintered body and may cause deterioration of ceramic properties, some attempts have been made to precisely control them. For example, slow drying and the addition of large amounts of binder have been effective in suppressing cracks and deformation to some extent. However, these efforts lead to prolonged drying times or the occurrence of cracks and deformations during dewaxing. Thus, it was not possible to suppress cracking and deformation during drying. This is because even though cracking and deformation are believed to be caused by inhomogeneous internal structural changes of the slurry during drying, they have not been understood, and therefore, not reasonably controlled. To solve this problem, it is necessary to accurately understand what occurs inside the slurry during drying, but there has been no such research to date. Optical coherence tomography (OCT) is a novel technique to observe the internal structure of opaque materials at high speed and high resolution using light interference. The objective of this study was to develop a new system that combines OCT with thermogravimetric analysis (TG) consisting of an infrared heater for heating and an electronic balance, which enables operando observation of internal structural changes of ceramic slurry during drying, and to reveal the inhomogeneous changes in the internal structure in the SiO₂ slurry during drying. We prepared a SiO₂ slurry composed of spherical SiO₂ particles and water. The SiO₂ particles in the slurry was confirmed to be well dispersed by flow curves without hysteresis character. The prepared slurry was formed into sheets using a doctor blade machine, thereafter, the internal structural changes and weight changes during the drying process were observed while the ambient temperature was controlled at 50?. In the early stages of drying, the OCT signal intensity changed significantly owing to the Brownian motion of the SiO₂ particles in the slurry. The slurry was found to shrink as it dried and then lose its fluidity, and the ceramic particles were found to agglomerate from the surface. In the final stages of drying, localized dry regions developed in the vicinity of the slurry surface and expanded with drying. Crack formation was also observed in a part of the localized dry regions. Therefore, crack formation was attributed to inhomogeneous drying-induced shrinkage and stress generated in the drying object during the progression of localized drying. The novel operando observation technique using combined OCT-TG is successfully demonstrated to be an effective approach for understanding the internal structural changes in opaque slurries during their drying.