This paper proposes a new voltage control scheme for distribution networks based on a multi-agent system. Multiple On-Load Tap Changing transformers including SVRs, each of which regulates multiple points of voltages, are optimally controlled. Large voltage fluctuations caused by PV generations are effectively regulated. We achieved an optimal control performance by developing an optimal control law that is applicable to decentralized autonomous control. The autonomy is accomplished by using a multi-agent system instead of the centralized control scheme. The effectiveness of the proposed scheme is demonstrated through numerical simulations with successful results.