According to one embodiment, an operation planning system includes a demand information database and a plurality of planning units. The demand information database stores demand data including an overall output required for a plurality of operation units for a planning period in an operation plan. The plurality of planning units adjusts output settings of the plurality of operation units. Any of the planning units selects a combination of operation units which provide the overall output required for the planning period, and based on operation costs depending on the outputs of the operation units, a process is repeated to generate the operation plan, the process including: selection of the operation unit belonging to the combination of operation units; and adjustment of the output setting for the selected operation unit by the planning unit paired with the selected operation unit.