Optimization technique to find a path of an inner reinforcement of an automobile hood is proposed by using design sensitivity informations. The strength and modal characteristics of the automobile hood are analyzed and their design sensitivity analyses with respect to the thickness are carried out using MSC/NASTRAN. Based on the design sensitivity analysis, determination of design variables and response functions is discussed. Techniques improving design from design sensitivity informations are suggested and the double-layer method is newly proposed to optimize the path of stiffener for a shell structure, Using the suggested method, we redesign a new inner reinforcement of an automobile hood and compare the responses with the original design. It is confirmed that new design improved in the frequency responses without the weight increasement.