1. A method for providing a game, over a communication network, to a plurality of user devices from a server device having a storage unit for storing user information relating to a plurality of users, the method comprising the steps of: (a) responsive to the user information for the plurality of users, generating a plurality of missions for each of the plurality of users; (b) storing in the storage unit a plurality of relations between a plurality of items and the plurality of missions; (c) transmitting over the communication network, to a first user device, displaying information for presenting a list of the missions generated for a first user on the first user device, the list indicating each of the missions, an item associated with a mission which the first user can acquire by clearing the mission, and an exchange element for changing a displayed mission to another mission to be presented in the list, wherein the exchange element is enabled based on at least one of the missions in the list being cleared; (d) receiving an identifier of an item from the first user device; (e) identifying a second mission in which the item specified by the received identifier can be acquired, responsive to the stored relations between the plurality of items and the plurality of missions; and (f) updating the displaying information so that said at least one of the missions included in the list is replaced with said identified second mission generated for the first user, when the exchange element is activated.