If we try to slove the problem of seepage through dams which have core walls, we are necessarily forced to clarify the true phases of seepage through dams which slant to the upper and the lower stream sides, especially free surfaces in the dam sections. Such free surfaces have quite different characteristics as compared with those in rectangular dam section. With regard to such free surfaces, no paper have ever been made public yet. This paper, however, explains this problem basing on the writers original idea.