This paper describes an operation control method for fuel-cell/battery hybrid power supply systems. Demand load power fluctuation is compensated by quickly charging and discharging batteries in these systems. State of charge (SOC) estimation based on battery management is essential for theses hybrid power supply systems. In this paper, we have proposed the control method for maintaining a constant value of SOC. Changing fuel-cell power and regulating load are controlled by using the SOC estimation method. We also have discussed the characteristics of the control method through simulation studies. These results clarify the effectiveness of the control method to maintain SOC.