The ability to continuously examine the interior hair structure throughout a treatment process is very important in designing effective hair products. Microscopy is commonly used to observe the interior of hair, but this method requires a sliced sample, making continuous observation impossible. Use of X‐ray computed tomography (CT) as a non‐destructive measurement has been proposed, but this method has a disadvantage in that it is impossible to obtain full‐color interior images of the sample. Thus, a non‐destructive method for continuous, full‐color examination of the interior hair structure has been lacking. In this study a new method is proposed that enables non‐destructive and continuous measurement of the interior hair structure with color information. In our method, optical CT is used for reconstruction of the interior hair structure. Our new theories enabled us to solve the crucial problem of the large observational error of traditional optical CT systems caused by internal light scattering and to make its practical application possible. A new optical CT system based on our method was implemented. This system displayed sufficient accuracy when the phantom image was measured, and clear and full‐color cross‐sectional images were obtained without destruction of the sample when human hair was observed. When the bleaching and dyeing processes were continuously measured, changes in the interior hair with time could be observed. These results clearly indicate that our new method provides a powerful tool for research and product development.