Group signature is one of the main theme in recent digital signature studies. Typical signature algorithm is a combination of more than 70 elliptic curve (ECC), modular (RSA), long-bit integer and hash arithmetic functions. A full H/W IP core is strongly desired for the use of group signature in SoCs in slow-clock and low-power mobile devices and embedded systems. Flexible adjustment of H/W speed and size, depending on different systems and LSI process technologies, is also required. However, for designing and verifying H/W, the group signature algorithm is too complicated to use a standard RTL (Register Transfer Level) design methodology nor any recent HLS (High Level Synthesis). Therefore, we incorporated a two-level behavioral synthesis approach, where an optimized macro-architecture is explored by a custom-made scheduler, after a database of multiple number of microarchitectures are effectively constructed by conventional HLS. We implemented the signature algorithm on a low-cost 0.25um gate-array. The H/W size is approximately 1M gates and our chip can compute a group signature at the equivalent speed (0.135 seconds@100MHz clock) with 3GHz PC S/W, while the power consumption is two orders of magnitude lower (425mW@100MHz).