The filtration process of gel particle slurry was experimentally investigated with three kinds of gel particles. Gel particles have a high water content and are able to compress themselves, and so build up highly compressible filter cakes by filtration. The hydraulic pressure distributions in the gel particle cake were successfully measured by using a subminiature pressure sensor for evaluating the internal behavior of gel cake. This process was able to be divided into four different periods of separation. In this study, the separation modes were hypothesized and the methods for theoretical calculations were developed. The results of the study are useful in obtaining proper understanding of the filtration process of soft and hydrous materials.