To understand the thermal diffusion that is generated when a stable temperature gradient is imposed on a binary liquid mixture, we propose an original system to measure Soret coefficients by using laser holography. By using this system, the development of the concentration profile accompanying thermal diffusion was visualized as an interference fringe. Local concentration could be evaluated from the interference fringe by employing the fact that interference fringes coincide with contour lines of reflective index, and that both temperature and concentration influence reflective index. The Soret coefficient could then be calculated using an analysis solution. The experimental values of the Soret coefficient agreed well with literature values.