Flexibility of the mechanical system is one of the critical factors of the bandwidth limitation of the positioning control. In this paper, we consider a manipulator mounted on the base which contains flexibility. Based on the positive realness of the transfer matrix, we define 'Robust arm configuration' which is a kind of singular configuration where the system is passive. We show that the mechanical system obtains good robustness in the neighborhood of the robust arm configuration. The validity is confirmed applying to a flexible assembly system. The position of the base and the pallets are optimized and a high bandwidth positioning is realized.