Slip casting is widely used as a relatively simple molding technique for complex-shaped ceramics. In slip casting, particles adhere to the mold through sorption of water from the slurry by the capillary force of the porous mold. Although it is known that the slurry dispersion affects the casting rate in the previous experimental and numerical studies, internal structure evolution during slip casting has not been clarified yet because there has been no technique to observe the inside of opaque ceramic slurries. Optical coherence tomography (OCT) is characterized by high resolution, high speed, and the ability to observe even opaque materials in three dimensions, making it suitable for solving the above problems. The objective of this study is to clarify the internal structural evolution of ceramic slurries during the slip casting process based on in-situ observation using OCT. In this study, 20 vol% Al₂O₃ particles were dispersed in water with a dispersant and a binder by ball milling. The slurry was poured into an acrylic mold on a porous Al₂O₃-ceramic plate, which is regarded as a one-dimensional model of slip casting. In-situ observation of the internal structural change of the slurry and the cake was performed using a sweeping OCT (IVS-2000-WR, santec Co., Ltd., resolution 3.4 µm) from the side of the acrylic mold. The cross-sectional observation of the slurry by OCT showed that the bright spots fluctuated in the slurry, while the bright spots were stationary in the cake layer. The thickness of the cake layer increased with time and the interface between the slurry and the cake layer moved from the mold surface to the upper side of the mold. Quantitative analysis of the internal structure evolution was carried out by motion detection technique using OCT images confirmed that the Al₂O₃ slurry evolution from a fluid state to a stationary state during the slip casting process and that the thickness of the cured layer is proportional to 1/2 power of time, which is good agreement with the Adcock-McDowall cured layer velocity equation. On the other hand, the attenuation rate of the OCT signal intensity, which is related to the water content, was estimated. As a result, the cake layer has lower attenuation rate, namely lower water content, and thickness of the cake layer increase as time. It is also clear that there is a casting transition layer between the Al₂O₃ slurry and the cake layer, in which the attenuation rate and motion pixel count, i.e., flowability and water content are gradually changed.