A player terminal displays a list including a plurality of candidate stamps and sends a stamp selected therefrom by a player to another player terminal. A stamp-metadata management unit of a server manages each of the plurality of candidate stamps in association with metadata indicating a certain situation in a game. A stamp-display-order determining unit controls the listing order for displaying the plurality of stamps on the player terminal on the basis of individual correlations between the plurality of metadata individually associated with the plurality of candidate stamps and situation data indicating a current situation in the game being executed. A metadata updating unit updates the content of the metadata for each of the plurality of candidate stamps on the basis of the result of selection of an item to send by the player.