1. A method for calculating, by use of a computer, design variables for a plurality of combinations of a plurality of operating states included in an operating range of an engine and for a plurality of combinations of a plurality of objective variables, the method comprising the steps of: obtaining global optimal solutions of the design variables that minimize or maximize a sum of the plurality of objective variables which respectively correspond to the plurality of combinations of the plurality of operating states; receiving designation of an upper limit or a lower limit of the plurality of objective variables; and searching for global optimal solutions that result in the smoothest change in the design variables when changing the operating states in a range of the received upper limit to the received lower limit of the objective variables, with one of the global optimal solutions as an initial value set.