A POS system includes: input means for receiving an input of an order; memory means for storing item information containing an individual theoretical weight of each item; calculating means for calculating, based on input information containing item types and item quantities of ordered items and on the item information, a theoretical total weight of the ordered items; weight measuring means for measuring an actual total weight of the ordered items, which are put in a bag; judging means for comparing the theoretical total weight and the actual total weight, and when there is a weight difference therebetween, identifying a side item and/or included item close in weight to the weight difference from among the ordered items; and output means for outputting the identified side item and/or included item as an item candidate corresponding to the weight difference.