To use effectively soft clays in construction works with high water content, a way which estimates the degree of strength increasing is suggested. As a strength of improved soft clays with pozzolanic materials depends on kinds of pozzolanic materials, to estimate the characteristic of strength increasing due to chemical reaction of silica and alumina which are chemical component of pozzolanic materials was tried. As a result, it is found that strength increasing can be expressed by the amount of (silica+alumina). Further, it is pointed out clearly that a degree of pozzolanic reaction and a improved strength differs from the amount of chemical component.