A heat source system managing device includes: a predicted heat demand upper limit calculating unit configured to calculate a predicted heat demand upper limit by adding a prediction error to a predicted heat demand value for a heat source system; an operation plan preparing unit configured to prepare an operation plan of the heat source system to supply heat of the predicted heat demand upper limit to a consuming facility; a surplus stored heat quantity calculating unit configured to repeatedly perform a process of calculating a surplus stored heat quantity by subtracting a heat quantity consumed by the consuming facility from the predicted heat demand upper limit; and an operation plan changing unit configured to sequentially change the operation plan by decreasing a future operation rate of a refrigerator to cancel the surplus stored heat quantity.