PURPOSE: To automate a change in the discount rate of each sales commodity by determining the variation rate of the discount rate and changing the stored discount rate based upon the variation rate.

CONSTITUTION: When the amount of a sales commodity is registered by register keys 51 on a keyboard and a sectional code including the sales commodity is inputted by a section key 52, a sectional table is searched, each information corresponding to the inputted section code is read out and the number (n) of digits of the registered amount is checked. Then operation for multiplying a reference variation rate by a value obtained by subtracting '1' from the number (n) of digits is executed to calculate the variation rate of the reference discount rate. Then UP/DOWN status is checked, and in the case of status=0, the calculated variation rate is added to the reference discount rate and the added value is set up as the discount rate of the commodity registered in the section. In the case of status=1, the calculated variation rate is subtracted from the reference discount rate and the subtracted value is set up as the discount rate of the commodity registered in the section. The discount operation of the registered amount is executed based upon the discount rate and a discounted selling amount is calculated.