The Public Key Infrastructure (PKI) technology is very important to support secure global electronic commerce and digital communications on networks. In electronic commerce, different PKIs need to be interoperated. Especially, they need to be merged for merger and acquisition of companies. This paper takes a different approach from cross-certification techniques toward merging different PKIs. Our method is to merge the multiple CAs into one. By using our method, the trust model with an efficient path processing can be built in comparison with the traditional merging methods. Then we formalize merging processes and discuss the certification path processing and the operational cost.