An auction system for providing an auction service via a communication network comprises an auction server machine and buying user terminals which are connected to the communication network; wherein the buying user terminals specify a single item or any combination of items for bidding when multiple items are sold as grouped items at the auction held by the auction server machine; and the auction server machine decides on successful bidders for bids made on a variety of combinations of the grouped items from the buying user terminals, such that the items composing the grouped items do not overlap to multiple bidders and the total price becomes maximal.