This invention relates to a method of measuring the internal structure (packing structure or dispersion condition of particulate material) of a composite filled with particles having an irregular matrix by observations based on its optical anisotropy, in which the internal structure (packing structure or dispersion condition of particulate material) of the composite obtained by mixing particulate material as raw material with a liquid material is made visible by utilizing the photoelasticity based on local rearrangement of liquid material molecules or difference of refractive indices of the particulate material and liquid material, and the structure thereof are observed, and an evaluation device using this principle of measurement.