The dewatering process of constant pressure expression was experimentally investigated with three kinds of bulk gels. While the structure of a gel is different from that of a solid liquid mixture, hydraulic pressure distribution in the gel approximately coincides with that calculated from the conventional theory of filtration during the first term of the process. This first term is able to be reasonably analysed as the filtration period. The subsequent process can be analysed as the consolidation period. Some of the liquid in the gel is mechanically inseparable, so as a rule it is essential to account for this liquid in analysis of the expression process for gelatinous materials. Results of electrical conductivity measurement of the gel indicate that the net free water almost corresponds with the water mechanically separable. The experimental results of agar-agar gel coincide favourably with the calculated results based on the conventional theory of expression and the compression permeability test data.