In a method for selling an approved material, the amounts of each material forwarded to and used by each customer are input into a computer, and the updated amount of inventory of the material in the customer is recorded. The computer checks the amounts of inventory and the used amount when issuance of an approved material certificate is requested from a customer having an ID number and a password through a network communication. When the used amount does not exceed the amount of inventory, issuance of the approved material certificate is approved, and the used amount is subtracted from the amount of inventory to update the amount of inventory. When the used amount exceeds the amount of inventory, the issuance is disapproved. When an approved material is sold and issuance of the approved material certificate is requested through a network communication, approval or disapproval is decided quickly and accurately.