For securing mobile ad-hoc networks, asymmetric key cryptosystem is one of effective technologies for encryption and authentication. However, for public-key distribution, the assumption about the availability of on-line trusted authorities or certificate repositories is unrealistic due to the very nature of ad-hoc networks. In this paper, a group-based public-key management system that allows users to generate public/private key pairs by themselves is proposed. In the proposed system, a group head issues certificates for group members and each node can perform authentication without requirement of on-line fixed trusted authorities. Taking self-securing approach, the proposal eliminates the problem of unrealistic assumption by introducing a localized trust model and suggesting an adaptive public-key distribution mechanism. Moreover, in the public-key distribution scheme, a solution for reducing overhead is proposed to adapt our system with the scalability of networks. Therefore, the system is suitable for any contemporary routing protocols in large mobile ad-hoc networks.