According to one embodiment, a seawater desalination plant system includes a plant unit that produces fresh water from seawater. The seawater desalination plant system predicts quality of seawater for each predetermined time (for example, 1 hour) for a certain definite period (for example, 24 hours) based on a past actual measurement value of quality of seawater; creates a plurality of proposed plans of fresh water production quantity during the definite period in each of which fresh water production quantity required for the definite period is allotted to the each predetermined time; calculates, with respect to the proposed plans, electric power consumption rates for desalination using the predicted quality of seawater, the fresh water production quantity and a previously obtained recovery rate; and obtains an optimum plan of fresh water production quantity with a minimum electric power consumption rate from the proposed plans.