1. A production instruction volume decision support method comprising the steps of: creating a plurality of sets of amount of the production order having information for specifying the production volumes of a plurality of products; simulating demand for each product from demand forecast information for each product; computing evaluation values of a plurality of evaluation indices from demand for each product and the production volume of each product for each set of amount of the production order; computing sets of amount of the production order which are inferior to other sets of amount of the production order in some evaluation indices but superior to the other sets of amount of the production order in the other evaluation indices to output a plurality of sets of amount of the production order; and displaying the output plurality of sets of amount of the production order so that a user can select one of them.