The method of determining a reserving order number and a final order number of a merchandise is performed by the steps of: computing a standard sales number D for each individual day of the week while averaging the sales performance for each individual day of the week with regard to the merchandise X which is an object for determining a reserving order number and a final order number thereof during a predetermined sampling period which are stored and updated in a memory and outputting the computed result; computing a first corrected standard sales number E for each individual day of the week based on a standard customer number correction coefficient C and the standard sales number D, for each individual day of the week; computing a second corrected standard sales number E.times..beta. based on a sales number variation coefficient .beta. and the first corrected standard sales number E for each individual day of the week and outputting the computed result; computing a reserving order number of the merchandise X for a concerned day of the week based on the second corrected standard sales number E.times..beta. it for each individual day of the week and an out-of stock safety coefficient .alpha. and outputting the computed result; and computing a difference between the sales performance of the day two day prior to the concerned day of the week and the reserving order number of the corresponding day and subtracting or adding the difference from and to the reserving order number for the concerned day of the week which is computed and outputted in the previous step to determine a reserving order correction number and outputting the determined result, whereby a method of determining a reserving order number and a final order number of a merchandise in a retailing business can be provided in which a loss of sales opportunity due to out-of stack of respective merchandises can be decreased, as well as with regard to daily delivered merchandises and perishable foods a loss due to freshness deterioration can be reduced.
