1. A device configured to calculate a battery usable time period of a mobile station, the device being provided separately from the mobile station, the device comprising: circuitry configured to retrieve information regarding usage conditions including operating time periods, the operating time periods indicating extents of utilizing corresponding plural functions included in the mobile station by a user; store, for each of operating modes of the mobile station, average consumed current values for the corresponding plural functions; calculate, for each of the operating modes, the battery usable time period by calculating an added value, the added value being obtained by adding a product of the average consumed current value for one of the functions and the operating time period of the one of the functions over the plural functions, and by dividing a capacity value of a battery included in the mobile station by the added value; control a display to display the battery usable time periods of the mobile station to the user; retrieve, for each of networks by which the mobile station was served, the information for the operating time periods of the corresponding plural functions from the mobile station; and calculate the battery usable time periods for each of the networks.