In a seawater desalination plant system, a seawater quality prediction means predicts quality of seawater for each predetermined time during a certain definite period based on a past actual measurement value of quality of seawater. In addition, an optimum plan calculation means creates a plurality of proposed plans of fresh water production quantity during the definite period in which fresh water production quantity required for the definite period is allotted to the each predetermined time, calculates, with respect to these, an electric power consumption rate for desalination using the seawater quality predicted for the each predetermined time, the fresh water production quantity, and a previously obtained recovery rate, and obtains an optimum fresh water production quantity plan from the plurality of proposed plans of fresh water production quantity based on this. A display means displays the content of this obtained optimum fresh water production quantity plan.