A server calculates, for each of a plurality of vehicles, a total number of chargers within an accessible range of a vehicle to calculate a reduced number so that the greater the total number of chargers is, the less the reduced number is, the reduced number representing a likelihood that the vehicle would arrive at each of the chargers. The server calculates, for each of the at least one charger, an effective number of vehicles which may use a charger to conduct external charging, by summing reduced numbers. The server calculates an index indicating a potential demand for external charging at the at least one charger based on the effective number.