1. A method of calculating a global optimal solution of design variables which minimize or maximize a combination of a plurality of objective variables for a plurality of combinations of a plurality of operating states included in an operating area of an engine using a computer, comprising: a step of obtaining local optimal solutions of the design variables which minimize or maximize a combination of the plurality of objective variables which are calculated for each of the plurality of combinations of the plurality of operating states; a step of obtaining initial values of a combination of the plurality of design variables used for calculating the global optimal solution; and a step of calculating the global optimal solution by solving, using a genetic algorithm based on the local optimal solutions and the initial values, an equation, which should be satisfied by the plurality of design variables.