The optical performance of thin film polarizers is highly sensitive to the layer thicknesses of thin film. The thicknesses of the sensitive layers are optimized in order to gain broader polarizing zone in such case when the total layer thickness does not increase. An automatic layer thickness control system is established, and errors caused by different monitoring methods are analyzed. With this thickness control system, thin-film polarizers with T_p higher than 98% and T_p/T_s higher than 200:1 (T_p and T_s are transmissions for p- and s-polarizations, respectively) with the bandwidth of 11 nm are prepared. Using the system allows for optimum repeatability of three successive runs.