This paper proposes a nested (hierarchical) SPN structure and the symmetric block cipher "Hierocrypt". In the nested SPN structure, lower-level SPN structures are recursively embedded into S-box positions in SPN of the higher level. This structure recursively assures the lower bound of active S-box number, and high security level is efficiently realized. The 8-round Hierocrypt is implemented in C language on Pentium III, and shows the middle-class performance of final AES candidates.