A new design method of the flux observer for the vector control is proposed in this paper. The newly designed observer estimates the flux within predetermined flux angle deviations. The characteristics are studied by Frequency Response Function (FRF) and the proposed designed method is shown to realize robust flux estimations. Then, the proposed observer is compared with the adaptive identifier, and they are shown to compensate with each other. Moreover, the proposed designed method is implemented in the vector control system and is shown feasible experimentally.