An optimization technique for the placement of a series of pressure sensors to be installed on the surface of the BFS structure is proposed. A state estimator based on LSE(least square estimation) using the surface pressure measured by the sensors is designed for the estimation of the eigen-modes of the density in the flow field. Numerical optimization minimizing the weighted mean-square-error of the states is formulated for the optimal sensor position. By comparing the optimum solution with the considerable positions by intuition at which the power strength of POD modes are maximum, we summarized the policy for the sensor placement from the flow control point of view.