The purchaser client of the corporation or the individual who is the contract member participates in the marketplace that is constructed in the server and requests for the commerce transaction of the commodity purchase. When the desired purchase commodity is not obtained in the marketplace, the server requests other marketplace with which the mediation contract is concluded for the commerce transaction of the commodity purchase by attaching the credit guarantee to the purchaser. With respect to the mediation with other marketplace, the credit enquiry is conducted based on the purchase history of the purchaser in his own marketplace and, when a predetermined estimation standard is satisfied, the commerce transaction of the commodity purchase is requested to other marketplace with the credit guarantee information.