A lead acid storage battery system for use in a natural energy utilization system, including: a lead acid storage battery; a storage battery state measurement unit for measuring a state of the lead acid storage battery; an SOC model representing the relationship between factors of the lead acid storage battery, such as current, voltage, and temperature, and a storage battery charge of state of the lead acid storage battery; and an equalized charge performance unit for performing equalized charge of the lead acid storage battery, wherein said equalized charge includes fully charging the lead acid storage battery, the lead acid storage battery system comprising: an SOC estimation unit configured to estimate a state of charge of the lead acid storage battery from the information measured by the storage battery state measurement unit and the information from the SOC model; characterized in that the lead acid storage battery system further comprises: an SOC transition DataBase configured to record how a state of charge of the lead acid storage battery makes a transition; an SOC transition history management unit configured to record, in the SOC transition DataBase, a value of state of charge estimated by the SOC estimation unit and to check an SOC transition situation; a degradation model representing an operation situation of the lead acid storage battery including a state of charge of the lead acid storage battery, and degradation; and an equalized charge optimal planning unit configured to plan an optimal method of performing the equalized charge based on the information on the SOC transition situation from the SOC transition history management unit and the information from the degradation model. an SOC estimation unit configured to estimate a state of charge of the lead acid storage battery from the information measured by the storage battery state measurement unit and the information from the SOC model; characterized in that the lead acid storage battery system further comprises: an SOC transition DataBase configured to record how a state of charge of the lead acid storage battery makes a transition; an SOC transition history management unit configured to record, in the SOC transition DataBase, a value of state of charge estimated by the SOC estimation unit and to check an SOC transition situation; a degradation model representing an operation situation of the lead acid storage battery including a state of charge of the lead acid storage battery, and degradation; and an equalized charge optimal planning unit configured to plan an optimal method of performing the equalized charge based on the information on the SOC transition situation from the SOC transition history management unit and the information from the degradation model. an SOC transition DataBase configured to record how a state of charge of the lead acid storage battery makes a transition; an SOC transition history management unit configured to record, in the SOC transition DataBase, a value of state of charge estimated by the SOC estimation unit and to check an SOC transition situation; a degradation model representing an operation situation of the lead acid storage battery including a state of charge of the lead acid storage battery, and degradation; and an equalized charge optimal planning unit configured to plan an optimal method of performing the equalized charge based on the information on the SOC transition situation from the SOC transition history management unit and the information from the degradation model.