An automatic sales tax declaration system includes a control center, a point of sale (POS) terminal at a shop, a declaration machine accessible to a customer, and a tax declaration card. The POS terminal and the declaration machine both communicate with the control center via a mobile telecommunication network. When a sale is made, the merchant collects the payment using the POS terminal, which records customer tax information on the tax declaration card and transmits merchant tax information to the control center. Subsequently, the customer uses the tax declaration card and the declaration machine to transmit the customer tax information to the control center. The control center calculates the sales tax based on the customer and merchant tax information. If the control center does not receive merchant tax information corresponding to the customer tax information, it concludes that the merchant may have attempted to conceal sales and evade taxes.