According to one embodiment, an operation planning system includes a storage and a plurality of planning devices. The storage is configured to: store information of operating devices and planning devices included in the operation planning system; store a total output required for a plurality of operating devices in a planning period of an operation plan; and store the operation plan including a combination of operating devices operating in the planning period and an output setting value for each of the operating devices in the combination. The plurality of planning devices is configured to: generate the combination satisfying the total output required in the planning period; determine the output setting value for each of the operating devices belonging to the combination; obtain combination data which is data of the combinations generated by the other planning devices; and generate the operation plan based on the combination data.