An approach to the simultaneous optimal design of structure and control system for large free-free flexible beam is presented. The flexible beam is modeled by the finite element method. And the reduced model of small degree of freedom is constructed by use of modal analysis. The tapered beam is considered so that the number of design variables is not dependent on the increasing number of finite elements. The width of several points of tapered beam and control gain are taken as design variables. The shape of beam and control gain are optimized simultaneously for the minimum weight of total structure including control system subject to the constraints of the magnitude of displacement of beam. It is shown that the simultaneous optimal design of structure and control systems is indeed useful.