1. A method of calculating a global optimal solution of engine-related 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, the method comprising: 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; obtaining initial values of a combination of the plurality of design variables used for calculating the global optimal solution; and 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, wherein each of the initial values is used as a resulting value that is obtained through summation of local optimal solutions of the plurality of objective variables corresponding to each other when arranged in ascending or descending order.