An air-conditioning system controller controls one or plural air-conditioning apparatus installed in a building, and includes a processor that determines an air-conditioning apparatus control command so that a preset evaluation index satisfies a preset condition in a preset control-target period under a preset constraint. The processor divides the control-target period into time sections, determines a room-temperature change permissible range within which a room temperature satisfies the constraint, determines a heat-load change permissible range, based on the room temperature, the room-temperature change permissible range, a heat load estimation, and a heat load to be processed by each air-conditioning apparatus, and determines, for each time section, as the air-conditioning apparatus control command, an operation frequency and start-stop of the air-conditioning apparatus, based on the heat-load change permissible range and an operation efficiency of the air-conditioning apparatus.