Horizontal centrifugal casting is widely used for production of water pipes, rolls and so on. Advantage of this process is to press molten metal against the permanent mold due to centrifugal force. Thanks to this, one can obtain fine structure and sound castings1). On the other hand, macroscopic segregation on tangential direction sometimes forms. The formation mechanism of macroscopic segregation has not well understood. This is because the solidification process itself is still uncertain since it is very complex2-4). Thus, in order to understand the solidification process in horizontal centrifugal casting an in-situ observation has been made in this study. Transparent organic substance has been used to simulate the metal solidification.