This report examines the mechanisms of the impermeability used in a new water-expansion rubber material of injected type. Until now, Impermeability of segment joint of shield tunnel was retained by increasing contact surface stress between segment by pressing. However, it is very difficult to retain the impermeability in the complicated joint plane. Then, a new water-expansion rubber material of injected type has been developed. we researched the mechanism of impermeability because it was indicated that the mechanism differed from conventional contact surface stress. As a result, we found a mechanism of impermeability by blocking effect. From this mechanism, it was possible to predict the critical impermeability pressure by a semi-theoretical equation based on the equation of the extrusion.