A purchase information collecting method collecting reliable purchase information about any kind of commodity using a network. In the purchase information collecting method, a server apparatus is used, which communicates with a seller computer and a purchaser computer, and which stores received sales information in the first storing part when the sales information identifies a deal about a commodity and is generated and transmitted by the seller computer. The server apparatus, when receiving purchase information, which consists of personal information of a purchaser of a commodity and the sales information that is presented to the purchaser by a seller of the commodity, judges whether the sales information included in the received purchase information is stored in the first storing part. If the sales information is stored, the server apparatus stores the received purchase information as valid purchase information in the second storing part.