A counting device includes processing circuitry configured to store rectangular area definition information indicating positions and sizes of rectangular areas obtained by dividing an area where a number of mobile devices is counted, count the number of mobile devices present in each of the rectangular areas indicated in the rectangular area definition information, every predetermined time, based on position information of each of the mobile devices, and when it is determined based on a counting result of the number of mobile devices present in each of the rectangular areas that there are rectangular areas where the number of mobile devices is smaller than a first threshold, integrate the rectangular areas to generate a new rectangular area, when it is determined based on the counting result that there is a rectangular area where the number of mobile devices exceeds a second threshold, divide the rectangular area to generate new rectangular areas.