To provide a system used for safely, easily and promptly executing settlement when a commodity or a content is purchased on the Internet or a cellular telephone communication network.

A middleman mediating settlement between a buyer and a seller issues an authentication ID usable only once and informs a buyer of it every time the buyer wants to buy a commodity or a content, and stores the authentication ID, and the price of the requested commodity or content in a database. The buyer inputs the authentication ID issued from the middleman to the seller instead of a credit card number or a membership number, and the seller refers to the middleman about the authentication ID inputted from the buyer and the price of the commodity or the content the buyer wants to buy, whereby safe and prompt settlement can be realized without requiring the buyer and the seller to have a special device or software.