Crystallization inside micro channel is key technology to making new materials. In order to control the quality of crystal, solubility observation is required. Interferometry is technique to measure solubility using refractive index but it requires complex optical layout. In this paper new technique to measure refractive index inside micro channel using digital holographic technique are proposed. Calculated particle positions are shifted because of variation of refractive index. With this shift, the refractive index inside channel canbe calculated. With simple experiment, feasibility of the technique are confirmed.