PURPOSE: To report the product that sells most among other related products when a particular product is out of stock.

CONSTITUTION: A PLU files 28-1 stores sales data, current stock amount and proper stock amounts for each product. Further it stores a link group code to which the product belongs and sales ranking in the same group. A CPU 21 updates, at product registration, sales data and current inventory in the PLU file 28-1 corresponding to the product. At that time, the CPU 21 compares the current inventory in the PLU file 28-1 corresponding to the registered product with a proper inventory so as to detect whether the current inventory has reached the proper inventory or not. Then, if the current inventory is less than the proper one, the other products which belong to the same product group and sell more are retrieved and their product code are displayed by message.