We have established an evaluation method based on optical coherence tomography (OCT), which uses light interference, as a method for three-dimensional in-situ observation of the internal structure of slurries, compacts, and sintered bodies. Nondestructive inspection by evaluating defects inside sintered bodies, dynamic changes in particle sedimentation and drying processes in ceramic slurries, quantitative evaluation of pore structures in dry forming under applied stress, and visualization of internal structures under high temperature in dewaxing and sintering are enabled. This method is expected to be of great help in research and development as an innovative evaluation method that enables consistent and direct observation of structural changes from the micrometer scale equivalent to particle size to the millimeter scale equivalent to bulk materials under a variety of actual environment conditions.