Directional growth experiments in a water-KCl solution were carried out using a new experimental apparatus combined with an optical system of a Mach-Zehnder interferometer, and the solute distribution in the solution in front of the ice/water interface, as well as the interfacial patterns, was observed in situ. This is the first study in which the solute diffusion field was obtained by analyzing the interference fringes. It was found that the solute distribution is strongly affected by the pattern of interface (i.e., the diffusion field of solute varies greatly with the development of interfacial patterns). On the other hand, observations of interference fringes inside the ice crystal showed that the crystal shape develops in three dimensions even in a thin growth cell. Interaction between the diffusion field and the interfacial pattern is closely related to the three-dimensional structure of the ice/water interface.