[en] Horizontal centrifugal casting process is widely applied for long products. However, it often has macro-segregation problems, such as band segregation. In order to understand solidification process in this process, in-situ observation using transparent substances has been performed. Here, the effect of liquid volume in the mold was investigated. Succinonitrile-water alloy was introduced in the glass cell and filling ratio of organic alloy was varied from 70 to 100 vol. % and the rotation rate was set to be 250 rpm. Equiaxed grains formed in case of low filling ratio. On the other hand, no equiaxed grain formed when the filling ratio was 100 vol. %. Strong fluid flow may be induced by the fluctuation of free surface and this may form equiaxed grains. However, when the area of free surface decreased with increasing filling ratio, the influence of free surface diminished and fluid flow was stabilized. Therefore, well-developed columnar zone grew and no equiaxed grains formed. (paper)