In the civil engineering field, water-swelling materials are often used as joint sealing materials to improve water cut-off performance in the joints of the steel (pipe) sheet pile. In short, water-swelling materials are extremely effective as water sealing materials. However, many characteristics of water-swelling materials have not yet been proved, and one of the unproven characteristics is the swelling pressure. Solving the issue of the swelling pressure will contribute to improving water sealing in the joints of the steel (pipe) sheet pile. At the same time, it is also expected to greatly contribute to the application of water-swelling materials for other purposes, or the new development and improvement of water-swelling materials.The purpose of this paper is to prove the characteristics of swelling pressure of water-swelling materials which are applied to joint sealing materials, e.g., a steel (pipe) sheet pile, etc., and to experimentally evaluate the swelling pressure of water-swelling materials.The results we obtained are : (1) the swelling pressure depends on the membrane thickness of the water-swelling materials. The thicker a membrane becomes, the bigger the pressure tends to be. (2) While the swelling pressure is not much influenced by the temperature of the water to be immersed, it is influenced by the quality of water to be immersed. Similar to the traits of the swelling rate, the swelling pressure when immersed in artificial sea water is lower compared with that when immersed in freshwater. (3) For example, water-swelling materials 1 mm thick achieve a swelling pressure of 6.7MPa when immersed in fresh water, and 4.2MPa when immersed in artificial sea water, respectively. Accordingly, we assume they should fully perform the swelling functions under a restricted environment as well.