As a basis for the development of energy storage systems utilizing crystallization/dissolution of chemical substances, an experimental study is performed for crystallization of ammonium sulfamate from an aqueous solution confined in a rectangular vessel, A molded polycrystal adhering to one of the vertical walls in the vessel was made to grow by cooling the opposite wall. The behavior of natural convection in the solution was visualized using granulated tracers. In addition, local changes in solution temperature were also measured, In the neighborhood of the growing polycrystal, upward flows develope through the decerase in concentration of solution due to crystal growth, as well as by the increase in temperature due to the Latent heat of crystallization. Dilute solution transferred upward is retained at the top of the vessel to form a horizontal rolling cell there. After that, ascending dilute solution is retained above the cell, and forms a second cell. Finally, horizontal multi-layers are established in the liquid phase. Moreover, the effects of cooling rate and flow structure on the crystal growth is discussed.